Fix is_date_field for simple types

Having a simple type returned from e.g. a hybrid_property, an
`AttributeError` would be thrown from `is_date_field`. Fixed by checking
for the `property` attribute before accessing it. Added test case that
provoked the error prior to the fix.
Allow post/patch of nested submodels

Recurse into nested relationships and find/create models on post/patch
(previously threw TypeError: unhashable type: 'dict')
Changed for readability
pep8 fixes
Notes changes due to previous commit.
Merge pull request #220 from logandk/hybrid_setter_error

Fix is_date_field for simple types
Notes changes due to previous commit.
Decapitalizes issue descriptions in changelog for consistency.
Prepares for release of version 0.12.0.
Bump version number to 0.12.0
Set development version number to 0.12.1-dev
Merge pull request #222 from logandk/add-related-error

Allow post/patch of nested submodels
Notes change due to previous commits.
Adds files to allow building of PDF documentation.
Extract model pk before postprocessors are called

This prevents problems when the pk field is renamed, removed or moved to a nested dict in a postprocessor.
This is punishment for writing a PR in the github editor :-)
Merge pull request #241 from joostdevries/master

Extract model pk before postprocessors are called
Adds pysqlite package to test requirements
Return a sinlge object if relationinstid is given in API.get
Modify the order of postprocessor