Layers API: Create/Update/Delete support

Summary:
Refactor layers GET requests to use Ushahidi_Rest and Endpoint
Add layer Read/Search Specs

Add Create/Update/Delete endpoints for layers

Test Plan:
`bin/phpspec run spec/Ushahidi/Usecase/Layer/`
`bin/behat -f progress -c application/config/behat.yml application/tests/features/api.layers.feature`

Reviewers: aMoniker, shadowhand

Reviewed By: shadowhand

Subscribers: bodacea

Maniphest Tasks: T53

Differential Revision: https://phabricator.ushahidi.com/D372