Add order argument to support custom ordering for categorical values

[tompollard]

This pull request includes several changes, including:

• add order argument, that allows the user to define custom orders for categorical variables (e.g. `order = {"ICU": ["CCU", "MICU"]}. Fixes #83.
• alters the limit argument to support a dictionary for different variables (e.g. limit = {"ICU": 2}). Fixes #59 by allowing the user to specify a limit of 1 for binary variables.
• column names are now strings by default (previously columns grouped by numerical values would have a numerical type).
• tableone.ipynb has been updated to reflect the changes above.
• minor style changes (e.g. tidying for PEP8).
• version bumped to 0.6.6

[tompollard]

@alistairewj or @jraffa please could you take a quick play around with this to check that the changes work as expected before I merge?