Don't put MERGIT comments around initial file.

The command-line tool bypassed this problem, but
the Ruby classes didn't. :-(
Refactored processor tests to use example files.

Instead of using code files as examples, use explicit test
example files.
Refactored processor tests: subject created via lets()

Now the subject is created entirely via let() statements
which makes reading the tests a little easier.
Overhauled how scan_file() and find_requirement() work

* find_requirement() is only called by scan_file()
* scan_file() returns true or false
* The MERGIT comments are saner.
* Improved the test coverage.