Add filters to App base class

Closes #6

- Change extend helper to wrap life cycle events
- Add filter registration to App base class
Merge pull request #8 from jgable/addFilters

Add filters to App base class
Ensure filters get default priority

- quick fix for filters where priority is not defined to ensure that
Ghost assigns the default priority
- there will be a matching commit in Ghost
Ensure filters ALWAYS get default priority

- fixing up my previous half-assed change + tests
- BAD HANNAH
Adding Travis badge to README.md
Switching from underscore to lodash

closes #11
Rename unregisterFilter to deregisterFilter

fixes #12
- Rename unregister to deregister
- Adjust tests
Merge pull request #13 from halfdan/deregister

Rename unregisterFilter to deregisterFilter
Bump version to 0.0.2