Decouple the graph and render logic from the fs watcher

[xzyfer]

This logic is all tightly coupled in the bin. This logic has been
notoriously impossible to test due to weird fs timing issues. By
extracting the actual logic we're now able to test it in isolation.

With this in place replacing Gaze (#636) becomes a viable option.
This PR not only massively increases our test coverage for the
watcher but also address a bunch of known edge cases i.e. orphaned
imports when a files is deleted.

Closes #1896
Fixes #1891

[xzyfer]

This is a super older version but it's the last version that supports Node 0.10. It's only for tests so I'm not too fussed.

[nschonni]

Looks good to me. But here is an alternate take/suggestion:

1. Abstract all the gaze.xxx calls in bin/node-sass to functions in the watcher
2. Extract the watcher to a new NPM module

[xzyfer]

Abstract all the gaze.xxx calls in bin/node-sass to functions in the watcher

Can you elaborate on how this differs from what I've done here?

Extract the watcher to a new NPM module

This is related to #1056. I'm open to it.

[nschonni]

Abstract was probably the wrong word, but here is a crude example example

    var watch = watcher.watched()
      if (watch.indexOf(file) === -1) {
        watcher.add(file);
      }
    });

So the watcher module would export certain functions required in the bin

[xzyfer]

Sorry I'm still not following.

If I'm understanding correctly wouldn't that then couple us to the fs watcher implementation's api?
What happens if the next fs watcher doesn't expose a method like watch() for exposing all currently watched files?

The watch.indexOf(file) === -1 is an optimisation to prevent calling gaze.add superfluously. Under the covers gaze.add also does deduping but there's not guarantee the next fs watcher would.

[nschonni]

The other part of splitting it would be a conditional require

var watcher;
if (option.watcher !== null)
  watcher = require(option.watcher)
 // error handling
} else {
  watcher = require('node-sass-watcher');
}

[xzyfer]

We could move this logic into lib/watcher but if we were to extract it into it's own package I can see uses cases for people wanting to poke changed partials i.e. url rewriting, or some kind webpack magic.

[xzyfer]

This could be

files.removed.forEach(gaze.remove)

but it's not likely the next watcher would have the same API

[nschonni]

I think this is probably fine to land as-is as it gets us testing.
Looking at the bin again, if we do a second pass, I think all of the gaze/watcher may just be pulled out by changing https://github.com/xzyfer/node-sass/blob/e81d6b9f225be4ea124a244d6e287a3711044e9f/bin/node-sass#L312

   if (options.watch) {
     watcher.handle(options, emitter);
   } else if (options.directory) {
     renderDir(options, emitter);
   } else {
     render(options, emitter);
}

Although I may be missing an obvious problem with passing it that way

[xzyfer]

Yeah there will be more passes. I also want to extract the console emiiter stuff because it's the cause of a lot of issues and feature request.