Update grunt watch so it does less work about plottable HOT 4 CLOSED

Fixed, with changes to grunt it only compiles the full project if core files change, and if test/example code changes it only recompiles that section. We can still do a little more cleverness; if we make a change to the core files that does not change the typing, then there is no need to recompile the examples and test code since contract is unchanged. We can test this by diffing the plottable.d.ts file before and after compilation.

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Right now our build (& times) looks like this:
compile core - 3s
compile test - 3s
compile examples - 3s

I suspect there is substantial overhead (about 3s) in running a seperate compile task, which means that compiling core, test, and examples is inefficient when we could just compile everything in around 3s in a single go. The reason we don't right now is that it's convenient to have all of the core compilation go to a single output file, but we want to send test compiled artifacts and examples compiled artifacts to different places. If we compiled everything to intermediate representation (eg axis.ts -> axis.js) and then concatenate the results together into the final script (plottable) then we would have a 3s full build instead of a 9s full build. The trick would be getting the scripts concatenated in the right order I think, right now the tscompiler is handling this for us. However, we know the order in which to put them (it is right there in reference.ts) so it would be as "simple" as writing a parser that uses a regex to suck it out of reference.ts and then concats them all together in that order.
@derekcicerone can you comment on whether this seems reasonable?

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Although we can always take the simpler approach - just implement the proposal above to check whether the d.t.s interface changed - in most cases we won't change the typing so it won't trigger the recompile of test or examples, so we get 3s compile is most cases and 9s compile only in the case where we modify the plottable API.

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I think you should use the concatenation support built into the TypeScript compiler. It already handles ordering of the files.

If you stick with the current tooling, consider breaking up the watch task into 3 separate targets if possible. Then each one will only run if necessary.

Beyond that I don't have much advice because I would solve this via IDE support - not Grunt.

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