Support Mamba BDD tests

[fatuhoku]

It'd be great to be able to run Mamba tests with PyBuilder.

Mamba is a BDD style testing library.

[fatuhoku]

A simple way out of this: #16.
Let the user run mamba with whatever flags they normally do; forward the test output to console. Observe only the process result. If there were test failures the process return code is set to a non-zero value. That should be enough to fail the build.

[mriehl]

Can now be done with

use_plugin("exec")

and

project.set_property("run_unit_tests_command", "mamba %s" % os.path.join(project.expand_path("$dir_source_unittest_python"), "spec.py"))