Added get-in function

To make getting items out of nested dictionaries/lists easier.
Tweaks to get_in

- Swapped order of first two arguments to allow for easier currying.
- PEP8 fixes.
- Added no_default argument.
- Added get_in to the __init__.py files.
- Improved the documentation with help from #93
- Tests should now pass on python 3.
Merge branch 'master' of git://github.com/pytoolz/toolz into get-in

Conflicts:
	toolz/dicttoolz/core.py
Merge pull request #126 from obmarg/get-in

Added get-in function