DEVEXP-131 User Agent

- setup a user agent string in urbanairship/core.py
- added nicer logging to urbanairship/push/core.py with __str__ override
- changed versioning to an easier to maintain setup with __about__.py
DEVEXP-131 updated user agent version

- read the version from an import, not from a file. much better
DEVEXP-131 Update

- removed unused variable in urbanairship/core.py
Merge pull request #1 from urbanairship/DEVEXP-131-user-agent

DEVEXP-131 User Agent, removed extra var, and merged
fixed version number error, no code changes