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

On Mon, Dec 29, 2014 at 05:01:14AM -0800, Joachim Durchholz wrote:
> This style would indent too far to the right

I don't think we have a proof for this...

> and it would also give the parameters that remain on the same line a
> special status

Why?

def foo(
  bar, spam,
  eggs):
    return 42

Has third parameter here a special status?  I don't think so...

>    My personal preference is that if the line gets too long, put the entire
>    parameter list to the next line(s).

Yes, this is a valid pep8 too.  We have a big number of options, see
https://github.com/sympy/sympy/pull/8538#discussion_r21469407, so
enforcing this rule shouldn't harm readability.

But in this particular case - this would be a bigger break
from the original formating (some parameters were on the first line).