Add offline mode (fallback to env)

[tochev]

I tried to add an offline mode using -o (as suggested in #6 ), however lein respected it only for the stuff from the maven repos while still downloading from clojars.
Also maven's dependency plugin outputted a lot of error messages with -o.
So I opted for defaulting to the fallback ($CLASSPATH).

Ideally this should be done in a more flexible way (e.g. falling back on cache, adding a command for updating the classpath, detecting when you are offline, etc.).

[tpope]

$CLASSPATH only is pretty worthless, no?

[tochev]

Unfortunately, pretty much so :( , unless the user is willing to set it "manually" when working on a project.
I wanted a way to be able to safely open random projects and also remove the downloading overhead.

As I've said, initially I tried -o but it did not behave the way I was hoping for.

[tpope]

I'm confused what you get out of this patch you wouldn't get out of just disabling classpath.vim entirely.

[tochev]

If I remember and understood the code correctly, vim-classpath is needed by vim-fireplace when running without connection to REPL.

My apologies if I misunderstood and feel free to reject the pull request.

[tpope]

Well it needs vim-classpath, but more importantly it needs the correct classpath it provides. If you feed it this default, it'll blow up for anything eval related at the very least. I guess :A and gf might work some of the time.