Extract TestSuite out into its own package to close #737

TestSuite is now in ``testing`` package. It has been discussed in #737.

### Changes
* [X] ``tests.go`` -> ``testing/testsuite.go``
  * [X] package ``revel`` -> ``testing``
* [X] Session
  * [X] ``cookie()`` -> ``Cookie()``
  * [X] ``getSessionFromCookie(...)`` -> ``GetSessionFromCookie(...)``
  * [X] update ``session_test.go``