Generic directives

Implemented a generic template for all Nagios object types. Adds an external dependency: `jinja2`.

Some trivial tests have been added (use `py.test` and `mock` libraries). Those tests are far away from complete, just added a minimal bunch of tests to verify I wasn't breaking anything obvious. No connections to PuppetDB are needed on these tests, method `get_nagios_data()` is being mocked.