API method TestPlan.create_test_run()

[atodorov]

As a follow-up to:
#444 (comment)
#444 (comment)

a new API method that will create a new TestRun from a TestPlan is requested.

NOTE: inside CreateTestRunView there are snippets of code which relate to the creation of new TestRun and then add the selected cases for execution to the TR. This needs to be refactored first and split into a method which can easily be reused both from the API and from the UI.

WARNING: this is a good candidate for a bounty but is a bit undefined ATM. If you are interested leave a comment and we'll refine the definition and tag it with a monetary label.

[atodorov]

Closed in favor of TestRun.create() & TestRun.add_case() methods.