Fixed readthedocs build

[danieljfarrell]

• Need to mock trimesh in conf.py because readthedocs cannot build RTree, a trimesh dependency.

• Updated setup.py to read __version__ from pvtrace/__init__.py

• Updated requirements.txt into three separate files

  • requirements.txt the main requirements file for installing and using pvtrace
  • requirements_dev.txt requirements for developing pvtrace
  • requirements_readthedocs.txt requirements for getting docs to build on readthedocs.

[danieljfarrell]

Merged pull-request #33