Changed default file name for generated files

Changed the default file name for generated CSS files from nodesass.css
to <originalfilename-without-scss>.css.

Now it should be possible to use globbing and get all generated files in
ones current working directory:

./path/to/node-sass path/to/styles/*.scss

generates all CSS files named like their SCSS counterparts.
Merge pull request #72 from dctr/master

Response to Issue 65