Add custom User Agent to every request, and expose the version in a readable way

[eengoron]

Closes #80

This PR:

• Changes the user agent sent on requests from python-requests/X to python closeio vY python-requests/X, where Y is the version of the closeio-api wrapper and X is the python-requests version
• Adds a __version__ variable to __init__.py to keep track of the version.
• Adds a helper to setup.py to read the __version__ variable from __init__.py to keep things consistent when installing the package.
• Updates the version from 1.1 to 1.2

I used the same structure as the Cleancat repo to do this.

[eengoron]

Tagging @tsx for review as well because of his work on Cleancat and ReleaseRabbit

[jkemp101]

I just did a little search about User Agent formats but we should confirm what format we want. Not sure if there is an RFC but I looked at this. That makes me think it should be something like this: Close/1.2 python (python-requests/2.21.0)

Edit: Or should it be Close/1.2 () python (python-requests/2.21.0)

[eengoron]

After talking with @jkemp101 -- I settled on Close/X python (python-requests/Y)