Use type validations to check consistency

Fixes #32 (I think)

As suggested by @ldodds. we're now using the `TYPE_VALIDATIONS` constant used for schemas to check cell formatting.

Currently checks for:

* Strings
* Integers
* URIs
* DateTimes
* Dates
* Times

This could possibly be improved by perhaps checking against common date / datetime formats and raise a warning if they don't match.