Added action to determine type of revision & Much More

1) Changed model events in RevisionableTrait.php and Revisionable.php
2) Added column 'action' to migration file.
3) Added 'action' field to revision to determine type of revision. Type of revisions include: 
* CREATE (i.e sql INSERT table row)
* INSERT(i.e sql UPDATE table column - from old null value to new non-null value)
* UPDATE (i.e sql UPDATE table from old non-null value to new non-null value)
* DELETE (i.e sql UPDATE table column from old non-null value to new null value)
* REMOVE (i.e sql DELETE table row)
4) Set $saveCreateRevision attribute in eloquent model to save revisions with action 'CREATE' when set to TRUE.
5) Set $revisionClassName attribute in eloquent model to provide human-readable name for Eloquent models.
6) Removed attribute 'updating' since we are now using 'updating' model events to determine when a model is being updated.

Todo:

1) Provide pre-formatted string for getRevisionString()
2) Provide a way to merge revisions of revisionable model and its relationships (which are revisionable as well).