Missing dependency (yaml)

[alekna]

PyYAML module missing in setup.py

Traceback (most recent call last):
  File "/usr/lib/python3.5/site-packages/v20/__init__.py", line 2, in <module>
    from v20 import account
  File "/usr/lib/python3.5/site-packages/v20/account.py", line 2, in <module>
    from v20.base_entity import BaseEntity
  File "/usr/lib/python3.5/site-packages/v20/base_entity.py", line 3, in <module>
    import yaml
ImportError: No module named 'yaml'

[FutureSharks]

PR to fix this: #25