minor: cleanup of rake task, improvements to guard file

- Cleaned up rake tasks, created 'spec' and 'spec:ci' which automatically
determines whether or not to push to coveralls or not.

- Disabled coverage checks for JRuby. This could potentially be enabled again
if we care enough to figure out why simplecov's reporting differs on the JVM,
but for now lets just supress this test in that environment.

- Improved/added to the Guardfile. I disabled the style checker in the rspec
guard so that it can run indpendently (we were previously running this twice
when files changed) and I added a guard for yard so that we can easily review
docs while we're working. I've also set guard to use the simple interactor for
JRuby to suppress an error with pry hooks.

- Updated the .gitignore file for yard (s/docs/doc).

- Remove Red Carpet from the gem file and replaced it with Kramdown which
works under all platforms.

Usage: bundle exec guard