Fix #2

This fixes the symptoms described in #2.

Pretty much, if there is a partial in the matched list, the compass helper (lib/compass) just drops out and never calls back: https://github.com/appleboy/gulp-compass/blob/master/lib/compass.js#L37 

The user can workaround this by avoiding selecting partials. That would go:

    .src(['assets/scss/**.scss', '!**/_*.scss'])

This patch should fix it: do callback instead of silently dropping out, and then drop from the list instead.

Sidenotes:

* I just discovered gulp, and I think I'm madly in love :-)
* given what compass is, we should really do an effort here to hide the c*** under the rug, and have it behave more "gulp" like - specially, an option like "css" should not be exposed and be used only internally as a way to hide file manipulations.