0.4.4: bugfixes in yaml serialization, error reporting, refactor internals

New Features

• DataFrameSchema provides rename_columns method #226 @baskervilski
• Failure case reporting is more intuitive as a tidy dataframe #232

Bugfixes

• yaml schema files support null index #225 @baskervilski
• add details to error reporting in type coercion #193 @c3-Anthony-Truchet
• fix column coerce warning in check_input/output decorators #228
• renamed internal decorator for check statistics #233 @Aditya1001001