Pull request adding a mega-test for the entire Rails Example Apps system, along with mega Rake commands

This pull request's main effect is *increased safety*.

By safety I mean the safety of all future contributions to gem r_a_c.

Because it increases the safety, this pull request should ease (and speed) development of r_a_c, especially by developers relatively new to it.

This pull request does this by supplying (to developers) an automated test of gem r_a_c's "full code flow".

Here is an explanation of this flow:

Gem r_a_c pulls in code from the Rails Example Applications during the process of regenerating them.

The only way to know for sure that changes to r_a_c do not break any Rails Example Application is by regenerating each example application and running its test suite.

To greatly ease this process, this pull request provides an automated Rake task (see [instructions](https://github.com/MarkDBlackwell/rails_apps_composer/blob/feature_megatest2/megatest/README.txt)) to do just that, by regenerating and testing all the Rails Example Applications.

Also it includes several useful new Rake tasks which operate (in batch) over the collection of Rails Example Apps (see `rake --tasks`).

These new Rake tasks work only for people running the Bash shell (and likely also the Bourne shell).

This pull request should not affect the ordinary users of the gem, nor its ordinary functionality, but is useful only to its gem developers.

Currently, the mega Rake tasks have been tested only in Rails 3. The purpose for this was to work through their issues more easily in the smaller world of just Rails 3 (and Ruby 1.9.3), just as a start.

This means (probably) there must be no Rails 4 gem available locally when running the mega Rake tasks (including the mega-test).

In Rails 3, the new mega-test passes. It regenerates eight (8) Rails Example Apps and runs their tests.

Actually, the mega-test will pass if certain pull requests (to the Rails Example Apps) are accepted (which I have submitted elsewhere to help them generate without error). :)

All r_a_c tests (`rake default`) also pass. (Originally #230.)