Add local modules, custom module directory and change module API

[jacobtomlinson]

The is a big PR which will close a few issues:

• Code
  • Change skill module imports (Change skill module imports #70)
  • Install modules locally (Install modules locally #45)
  • Specify module install directory (Specify module install directory #64)
• Tests and coverage
• Documentation

[coveralls]

Coverage increased (+0.5%) to 93.819% when pulling 878a090 on jacobtomlinson:local-modules into 64ee80c on opsdroid:dev.