Merge pull request #13 from nathancahill/functions

Add invoke(), for_each() and for_each_right()
Spacing.
Put comment on its own line.
Split tests into separate files.
Replace print function in tests with fixtures.noop to avoid print statements to console.
PEP8
Add once().
Add test for once().
Use class for after() object.
Update CHANGES with new functions added.
Add tests for for_each() and for_each_right() aliases.
Add partial() and partial_right().
Fix function reference.
Disable too-few-public-methods globally for pylint.
Add wrap().
PEP8
Add keys(), omit(), pairs(), pick(), transform(), and values().
Use callable() instead of checking for '__call__' attribute.
Add map_values().
Default callback to 1 for reset().