Tests for the new user_agent functionality.
First-pass user-agent implementation.

This was stolen wholesale from @kennethreitz's Requests module
equivalent, with only minor changes.
Pass through the user_agent function.

To match the README.
Pull out the implementation logic.
Add mock as a testing dependency.
Test CPython implementation.
Test PyPy implementation string.
Test unknown Python implementations.
Merge pull request #3 from Lukasa/user-agent

User agent constructor.