Fix setup.py so that new 'api' module directory is included in package

[svet-b]

The newly released v0.3.1 package cannot actually be imported after installation:

>>> import grafana_api
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/grafana_api-0.3.1-py3.6.egg/grafana_api/__init__.py", line 1, in <module>
    from .grafana_face import GrafanaFace
  File "/usr/local/lib/python3.6/site-packages/grafana_api-0.3.1-py3.6.egg/grafana_api/grafana_face.py", line 2, in <module>
    from .api import Base
ModuleNotFoundError: No module named 'grafana_api.api'

This is because the .api submodule is not actually included in the package. In order for it to be included, it needs to be explicitly listed in setup.py. This PR should address this.

[coveralls]

Coverage remained the same at 37.468% when pulling 0ea3f31 on svet-b:fix_setup into b9784a3 on m0nhawk:master.

[m0nhawk]

Thank you for help and fix on this!