PEP: pandas/core round 2
Fix layout and pep8 compliance for categorical, common, and frame.  Used YAPF with default settings and then manual modifications including:
-fixing line length of comments/docstrings for pep8 compliance
-converting lambdas to functions for pep8 compliance
-change YAPF default formatting of error strings
-change some YAPF line break choices to make them more natural in my eyes.  For example converting

```
if (function1(argument) and function2(
         argument)):
```

to

```
if (function1(argument) and
   function2(argument)):
```
