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 01, 2014 at 08:09:28AM -0800, Jason K. Moore wrote:
>    In .travis.yml:
> 
>  > @@ -116,11 +118,17 @@ before_install:
>  >        sudo apt-get update;
>  >        sudo apt-get install sagemath-upstream-binary;
>  >      fi
>  > +  - if [[ "${TEST_PEP}" == "true" ]]; then
>  > +      pip install git+https://github.com/jcrocholl/pep8.git;
> 
>    Seems smarter to install latest release from PyPi. What guarantees the
>    HEAD of master on their repo is not broken? I don't think we need the
>    bleeding edge on something like this.

That's somewhat temporary solution.  Latest stable release
has broken tests for E11* errors.  An example:

$ pep8 --show-source 
./sympy/core/add.py:110:25: E113 unexpected indentation
                        # e.g. x**2 -> 5   for ... + 5*x**2 + ...
                        ^
[...]