Add instructions for how to run the tests
Add get_branch_names method to report_generator

This returns all branches of all repos in the repository path,
passed as argument to the initialize method of ReportGenerator.
Add RSpec and BDD version of unit tests

Gemfile now contains RSpec, so it is included with bundle install.
The test_report_generator tests and test_helper are left untouched.
report_generator_spec and file_util do the exact same thing, but
with a different syntax. Note the comments in report_generator_spec,
they should not persist. Also the readme should be updated.
Correct readme instructions for running the tests
Use include matcher

Since position in the array is not important at this stage.
Improve semantics in context and spec descriptions
Copy contents of directory instead of directory itself