Signit

- Changed interface of `signit.signature.parse` and `signit.signature.verify`.
- Updated documentation and examples.

- Changed modules structure (https://github.com/f0t0n/signit/issues/3)
- Added basic documentation (https://github.com/f0t0n/signit/issues/4)
- Minor code refactoring

Minor release:
- Bug fix related to signature verification. Before this release it didn't work properly for signatures with custom prefixes.

Minor change:
- Exclude `pytest-runner` from the distribution

Minor improvements:
- Faster key generation (don't create new `SystemRandom` in loop)
- Faster tests at Travis CI (test runner command tuning)

First stable release.