Move extras_require to setup.py

Closes #573 (closed)

This MR breaks the optional requirements into a few files in order to read each as an option for extras_require.

Currently, users can do

$ pip install bilby[gw]

to install GW dependencies. This adds two new options

$ pip install bilby[mcmc]

which will install packages needed for bilby_mcmc proposals. There's also an

$ pip install bilby[all]

which will attempt to install all implemented samplers along with GW requirements.