Optional python binding with pybind11.

[MiaoDX]

Propose using pybind11 for the python binding instead of the boost.python.

Pros:

• boost.python is really hard to build on windows (and maybe others) platform and especially when we want to have python bindings for various python versions, what's more, it is really large.
• With pybind11, the grammar is almost very the same as boost.python but much more lightweight, we can even embed it into the source code (as shown in this PR)
• we get python2/python3 support seamlessly and no extra effort
• we can potentially publish opengv as standalone library on pip or conda

This PR can be reshaped since:

• There are both boost.python and pybind11 (in python_pybind folder) at present to be chosen by the user, maybe one is enough (you know, I mean the pybind11 one :) )
• What I did is almost replace bp and bpn with py, maybe there can be more advanced usage of pybind11
• The test is just passing the test.py in python folder, more tests are better

Anyway, hope this can be helpful.

[paulinus]

@MiaoDX, the python binding have been ported to pybind in #79. I think we can close this PR

[MiaoDX]

Yes, of course, one cleaner codebase looks better!