Cause less noise in the OS reading files: curdler.guess_file_type()
Refactoring curdler.unpack()

The following functions were extracted out of the `unpack()` helper:

 * find_setup_script()
 * get_setup_from_package()

A good amount of tests was writen as well.
Fix parsing requirements extra sections in `util.safe_{name,requirement}`
Tests for util.is_url()
Fix yielding extra dependency sets. Refs #44
Do not duplicate a test name
Rewrite the curdler.Script class as a function: run_setup_script() with tests!