100% test coverage, issue 82, refactoring, documentation

[snd]

• various improvements from refactor library contracts #82 and more
• add tests for Helpers library
• more docstrings
• 100% coverage (every branch and require in contracts is hit by at least one test)
• replace implementation of toString (now called uintToString) with one that works correctly for 0, integers with more than 8 digits and also uses less gas (before: https://travis-ci.org/paritytech/parity-bridge/jobs/328970765#L1482, after: https://travis-ci.org/paritytech/parity-bridge/jobs/328980140#L1466). the new implementation is thoroughly tested

[coveralls]

Coverage increased (+0.07%) to 96.503% when pulling 06e363d on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+0.07%) to 96.503% when pulling 3dcab58 on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+0.8%) to 97.203% when pulling 805b0d9 on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+1.5%) to 97.902% when pulling 3f1d495 on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+2.2%) to 98.601% when pulling 4fec6a4 on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+2.2%) to 98.621% when pulling f400cca on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+2.2%) to 98.592% when pulling 1634dc7 on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+2.9%) to 99.296% when pulling 6a052c1 on snd-issue-82 into bd5d0ae on master.

[coveralls]

Coverage increased (+3.6%) to 100.0% when pulling 80d6764 on snd-issue-82 into bd5d0ae on master.

[debris]

awesome! 🙌