Initial implementation of git commits

Implement unit tests, models, and migrations for commits and
repositories. Implement basic validations and associations for commits
and repositories.
Add commit association to tasks
Add DB constraints and inverse relationships

Add not null DB constraints and inverse_of relationships to commits and
repositories.
Fix association between tasks and commits

Debug has_many through association between tasks and commits. Add unit
tests to commits and tasks.
Initial implementation of nested attributes from tasks to commits
Add ER diagram
Initial implementation of commit-associated tasks

Implement commit nested attributes in controller. Modify task view form
partial to accept commits and associated repositories. Add URL column to
repositories. Modify task show view to provide full commit URL.

Delete unneeded controllers and assets for intervals.
Refactor nested_attributes in task model
Add Cocoon and refactor commit sub-form
Resolve infinite loop in task form

Resolve infinite loop in link_to_add_association in the task form.
Improve styling of task form commit sub-form.
Add CRUD for repositories

Add create, index, and delete controller actions for repositories.
Implement one view for all functions.
Merge branch 'gitcommit'

Conflicts:
	app/models/task.rb
	app/models/user.rb