Default strategies for more Django model field types

[Zac-HD]

There are several types of fields that we don't have a default strategy for, and it would be good to fix that.

Notes: some fields can just be added to the default mapping; others have extra metadata (eg length of strings, digits of decimals) to exploit. All exceptions should inherit from HypothesisException. We shouldn't try to generate autofields or file-related fields. There's some scope for reorganising the code (eg exception classes at the top, alphabetise fields, etc).