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

This has three forms that are acceptable:

Alignment:

```python
expr = _replace_mul_fpowxgpow(expr, sin, cos,
                                _midn, tan, _idn)
```

or hanging indents

```python
expr = _replace_mul_fpowxgpow(
    expr, sin, cos, _midn, tan, _idn)
```

or any number of indent spaces

```python
expr = _replace_mul_fpowxgpow(
  expr, sin, cos, _midn, tan, _idn)
```

See https://www.python.org/dev/peps/pep-0008/#indentation

I don't see this rule enforcement as a big deal. How is this going to cramp your coding style?

Sometimes we just have to give one up for the team. I think enforcing pep8 is a good idea, and yes we all will be annoyed with one little detail or another for a little while, but in a year, you'll probably forget that annoyance ever was there.