ENH: Add generic get_data_path function.

CPython docs warn that inspect.currentframe "relies on Python stack
frame support in the interpreter, which isn’t guaranteed to exist in all
implementations of Python." However, I tested this in PyPy too and it
works. Yeah!
STY: pep8
TST: Replace get_data_path
DOC: typo
DOC: error in README
TST: numpydocstringify
DOC: add module documentation
TST: Add test
STY: blank line
DOC: Improve info of returned string, and added a note about file existence.
Shebang and future import (per @ElBrogrammer's comment)
TST: allow running module
Merge pull request #263 from Jorge-C/get_data_path

Get data path