Add version method

[tjburch]

Rehashing #295 with a less messed up branch. Note the version is 2.2.5 already ahead of the next bump.

Replicating that text:
Following some of the annoyance in #281, I thought it might be good to introduce a module level version method like a lot of packages out there have. This adds that to a new file called pybaseball/version.py, which we should increment at every release. It's loaded in pybaseball.__init__.py so you can do the following:

>>> import pybaseball
>>> pybaseball.__version__
'2.2.5'

Then lastly, setup.py loads in the value from that file rather than internally defining it.