Add basic source map support

The scenario in which this works correctly is one where
the source file, compiled file, and source map are all in the same
location. Additionally, since this plugin will stream two files
for every one input, subsequent commands may need to pass through
when the source map is streamed.
Create a new vinyl File for sourcemap

...instead of cloning the source and modifying it. As pointed out by
@Contra, cloning results in copying the original file buffer
unnecessarily.
Remove compiled fixtures

Instead, fixtures compile along with the stream and the the results
compare against each other. This is a bit of futureproofing for
inevitable changes to coffeescript compiler output.
Merge pull request #8 from lettertwo/sourcemaps

Add basic source map support