Adds wait_for ajax helper to test suite.

The markdown tests were intermittently failing when the ajax
response containing the rendered markdown content of the ticket
details wasn't returning fast enough. The default Capybara
synchronization mechanisms weren't applicable in this use case.
The new wait_for helper added solves the problem.

See the note in the spec/support/markdown_shared.rb for details.
Turns unneeded instance variable into local variable.
Tweaks that make it easier to run the project locally.

Removes the mysql db backend replacing it with sqlite. Adds some
additional seeds to have something a bit more meaningful to look at
after firing up the project for the first time. Adds some additional
how-to's and context to the project README.