Remove unnecessary file
set_the_flash => set_flash
Deprecate set_the_flash in favor of set_flash
Reorganize set_session tests
Bring set_session in line with set_flash

* Deprecate `set_session(:foo)` in favor of `set_session[:foo]`
* Make both #[] and #to optional (`should set_session` simply checks
  that the session is non-empty, `should set_session.to('value')` checks
  that *any* value is 'value')
* Add support for regexps to #to
* Deprecate `should set_session(...).to(nil)` in favor of using
  `should_not set_session`
* Improve failure message