Filters Manage Dependencies

* Add dependency management test cases for each `Filter` with a
  dependency
* Add `assert_dependency_management_error` custom assertion which
  asserts a custom exception and message are raised if a dependency
  is missing
* Move all `Filter` dependencies to `Gemfile` `:test` block for
  test cases and CI
* Implement `TestingDependency` helper to abstract unloading and
  loading `Gemfile` `:test` block gems when asserting dependency
  management errors
* Implement `MissingDependencyException` custom exception with
 `MESSAGE` constant as a format string, so each `Filter` raises a
  uniform exception
* Add `begin..rescue..end` blocks around each `Filter` `require`
  statement to raise a `MissingDependencyException` when a gem can
  not be loaded
* Update README.md detailing new dependency management with listing
  of `Filter` gem dependencies
* Add gemspec post install message to inform users their apps must
  bundle `Filter` dependencies
Address Pull Request comments:
* Copy edit README.md
* In gemspec, link README.md dependencies and update post install message
* Rename test helper to `assert_dependency`
* Remove `TestingDependency` in favor of stubbing `Kernal#require`
Textile Filter does require RedCloth
Move ActiveSupport::XmlMini_Nokogiri to test helper, since its mixed in #to_hash is only used by #assert_equal_html
Merge upstream/master; activesupport is a full dependency
Relative link to Gemfile
Merge remote-tracking branch 'upstream/master' into simple-dependency-management
Don't test dependency management since its testing require and is brittle
No need for MissingDependencyException
If a dependency is missing, exit with a 1 status and write help message to STDERR.
Merge pull request #80 from simeonwillbanks/simple-dependency-management

Filters Manage Dependencies