Adding support for Hammer Medium actions.

* Methods for all hammer medium actions;
* Two simple tests;

This handles Issue #44.

```bash
$ python robottelo_runner.py --tests tests.cli.test_Medium
test_create_medium_1 (tests.cli.test_Medium.Medium)
Successfully creates a new medium ... ok
test_delete_medium_1 (tests.cli.test_Medium.Medium)
Creates and immediately deletes medium. ... ok

----------------------------------------------------------------------
Ran 2 tests in 6.903s

OK
```