Test runner able to run specific tests

I decided to make test paths relative to `ejtp.tests` module. Runner now use argv parameters to run specific tests. Each parameter can be a module, a TestCase or a test function. If you don't provide parameters, it just use test discovery to run all tests.

Fixes #107.

    $ python -m ejtp.tests.runner test_address
    ....
    ----------------------------------------------------------------------
    Ran 4 tests in 0.001s

    OK
    $ python -m ejtp.tests.runner test_address.TestPyAddress
    ..
    ----------------------------------------------------------------------
    Ran 2 tests in 0.001s

    OK
    $ python -m ejtp.tests.runner test_address.TestPyAddress.test_with_list
    .
    ----------------------------------------------------------------------
    Ran 1 test in 0.001s

    OK
    $ python -m ejtp.tests.runner test_address test_config
    ........
    ----------------------------------------------------------------------
    Ran 8 tests in 0.007s

    OK