Added configuration to allow for custom application engine.
Make code version specific for rails 3.1 and up

This commit adds Rails version checking to the RSpec application
configuration to handle Rails 3.0.x. The Rails::Engine class, while
available in Rails 3.0, does not have the capability to draw routes as
it does in 3.1 and above.
Tweak alternate app integration

This commit removes most if-statements from the example groups,
consolidating to a global method called `at_least_rails_3_1`. It
also adds filters to the rspec tests and changes engine_support.rb
to application.rb.
Removed the extra spaces between methods. They have been mocking me for months.
Merge pull request #566 from bnmrrs/patch-1

OPTIMIZATIONS! Removed extra spaces between methods
Raise error for Rails 3.0

Adds an override of Rails.configuration.application=(*) setter to
raise an error when Rails.configuration.application is set in Rails
3.0. Also moves at_least_rails_3_1? method to the RSpec::Rails module
as a public module method.
Merge pull request #539 from danrasband/custom-application-configuration

Added configuration to allow for custom application engine.
Move "at least rails 3.1" method to rspec/rails file.
This should be a writer method spec.
Move the "drawing of the routes" for the EngineExample to engine_example.rb
Don't need a file for this.
Don't need this example - it spec's an accessor.
changelog
No need for explicit return statement.
Nothing "custom" about this.
Move the "application swapping" to an `around` filter in the spec_helper.
After thinking about it, there is nothing "engine specific" about any of this new code. Therefore, no need to have engine specific specs.
Better changelog message.
convert matchers to subclass BaseMatcher class
revert all changes for #539

- not ready for release so backing out
- changes are stored in engine-support branch