Isolating engine

Isolating the engine to use StaticContent namespace in its class names,
in order to be able to couple the engine to projects *without warnings*
Rails 4 Support

Removes attr_accessible from model (since this responsibility has been
moved to controllers on rails 4)
Changes model to use rails 4 style on using
where(...).first_or_initialize instead of find_or_initialize
Changes tests to pass without mass assignment errors
Fixes #5
Fixes merge conflicts

Changes included by rails4_support branch and
changes included by isolate_engine branch