Ant style file matching.

Produces an iterator of all of the files that match the provided pattern.

Directory specifiers:
** matches zero or more directories.
* matches any directory name.
/ path separator.

File specifiers:
* glob style wildcard.

Examples:
**/*.py recursively match all python files.
foo/**/*.py recursively match all python files in the foo/ directory.
*.py match all the python files in the current diretory.
*/*.txt match all the text files in child directories.

>>> from fileset import *
>>> list(Fileset(".", [includes("*.py")]))
['fileset.py', 'test_fileset.py']