State now stores individual app definition versions instead of appRepositories per app
Minor cleanup.

- Removed unused class `History` and associated tests.
- Added docblock comments to `AppRepository`.
- Various minor formatting fixes.
Merge pull request #187 from rbpark/master

State now stores individual app definition versions instead of appRepositories per app
Merge pull request #192 from mesosphere/rbpark-master

Minor cleanup.
AppRepository.expunge now deletes current version

Fixes #194
Removed duplicate code and tests: ConstraintModule
AppDefinition validator now respects random ports

Renamed the validators used to reflect the more specific behavior they now implement.  Includes a new test in AppDefinitionTest.
Replace contributors with authors section in README

Rationale:

* Git already tracks this information
* GitHub has a "Contributors" tab that gives statistics based on Git
  history
* The section adds little value to consumers of the document
* A static list is difficult to maintain, which is why it historically
  has been out of date

Fixes #193.
Merge pull request #196 from mesosphere/validation-respects-random-ports

AppDefinition validator now respects random ports
Merge pull request #197 from mesosphere/contributors-to-authors

Replace contributors with authors section in README