Add methods for Table-related API calls

2 main api calls implemented

1. Add row
2. Update row

With these additions come a utility function to convert a row represented as a `dict` into `xml` for sending to BambooHR servers.
A couple of tests have also been added for the above mentioned 2 features, and also the generic get_tabular_data method.
The large chunk of text deleted and edited is just the `dict` used for test and is pure formatting changes (whitespace related) to adhere to PEP8.