Added stub / spec for executor interface

[cbednarski]

This is very basic but I think will give us a way to write a ulimit executor, cgroup executor, lxc executor, etc. This allows the drivers to be more OS agnostic and also lets us handle fiddly switching logic based on capabilities in the exec package instead of mixing it into the driver code where it gets mixed in with other logic.

Ideally exec stays low level and close to the OS, while the driver can assume that "run this thing with these constraints" just works using best-available techniques on the target platform.

/cc @catsby

Fixes #20

[armon]

@cbednarski Seems like a solid spec to use as a starting point

[catsby]

Could we support the idea of wrapping these?
Alternative question, could the Drivers retain knowledge of how to exec things, and we have things like ulimit context, cgroup context, so we compose a context in which the Driver executes?

I may be overcomplicating things

EDIT: I'm not actually sure this would even work, but wanted to throw it out there

[ryanuber]

I don't think this will compile on non-Linux, since this type is defined in a _linux.go file. Maybe instead the executors should register in somehow.

[cbednarski]

Ah, I will look into this. Registration makes things a bit more complicated because the write is racy and/or I have to reflect to copy the type from the registry (if you have a good example please link me!).

Here each goroutine gets its own (new) resource. The solution I have now is not elegant but it's simple and thread safe so if there is a compiler issue I will try to fix that for now.

[ryanuber]

I was thinking more like having a type ExecutorFactory func() Executor, then each executor could just use like it's init() method to add its own factory in. Or something like that. Whatever is easiest, I suppose.

[cbednarski]

Ah I see. That makes sense. Thanks!

[cbednarski]

I've tested this via the exec and java drivers and everything looks good. I'd like to add some more local tests but this is technically tested already. Also exec now works as non-root so this should not break running tests during development.

@catsby I'm going to merge this as-is but feel free to tweak if you need to for the cgroup stuff.

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