Add pep8 config, use pep8 in travis

also, fix E111-E113 errors:
E111 - indentation is not a multiple of four
E112 - expected an indented block
E113 - unexpected indentation

> Which rule enforces this?

E111

> This is one I don't like because

See [this](https://github.com/sympy/sympy/pull/8538#issuecomment-65062330), I don't think we can claim that our intend is to support PEP 8, if we randomly drop one rule of this guide or another.