Fix applySourceMap third parameter with source content

Set source content with relation to aSourceMapPath in
SourceMapGenerator#applySourceMap. Before, in the related test,
`../coffee/foo.coffee`'s source content would be placed at
`../coffee/foo.coffee` instead of where it should be at
`coffee/foo.coffee` derived maps. Not being stored at that location it
was left out of the generated map.
Merge pull request #100 from mzgoddard/fix-third-param-source-content

Fix applySourceMap's aSourceMapPath parameter source content support