No longer require pyyaml for 'setup.py install' or subsequently at runtime

'setup.py sdist' and 'setup.py develop' *do* require pyyaml,
but these are ua-parser developer scenarios.
Normal users who install ua-parser with Pip or Easy_Install
no longer need to install pyyaml.

Tested with:

- Sdist:
  * Fresh virtualenv
  * cd ~/src/ua-parser; git clean -dxf
  * pip install pyyaml
  * python setup.py sdist
  * cp -fprv ~/src/ua-parser/dist/ua-parser-*.tar.gz ~/.pip2pi
  * dir2pi ~/.pip2pi/

- Develop:
  * Fresh virtualenv
  * cd ~/src/ua-parser; git clean -dxf
  * pip install pyyaml
  * python setup.py develop
  * python -c "from ua_parser import user_agent_parser; print user_agent_parser.Parse('Chrome/29.0.1547.2')"

- Pip:
  * Fresh virtualenv; *no* pyyaml
  * cd ~/src/ua-parser; git clean -dxf; cd ~/src
  * pip install -i file:///Users/georger/.pip2pi/simple/ ua-parser
  * python -c "from ua_parser import user_agent_parser; print user_agent_parser.Parse('Chrome/29.0.1547.2')"

- Easy_Install:
  * Fresh virtualenv; *no* pyyaml
  * cd ~/src/ua-parser; git clean -dxf; cd ~/src
  * easy_install -i file:///Users/georger/.pip2pi/simple/ ua-parser
  * python -c "from ua_parser import user_agent_parser; print user_agent_parser.Parse('Chrome/29.0.1547.2')"