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

https://github.com/sympy/sympy/pull/8538#discussion-diff-21098044https://github.com/sympy/sympy/pull/8538#discussion-diff-21098044