Adds custom exceptions and associated tests
Reorganizes validator tests
Adds tests for min and max value validators
Adds documentation to custom validation errors
Adds additional numeric validator tests
Adds preliminary iterable validator tests
Adds additional tests for iterable validators
Merge pull request #24 from lyndsysimon/feature/validation_tests

Adds exceptions and tests for validators: type, value, length
Moves test fixtures from tests/validators/ to tests/
Incremental commit - overnight changes

Tests in subdirectories of /modularodm/tests/ have been written or
reviewed by me. Tests in /modularodm/tests/ were written by Nan or
Melissa, and I have not fully reviewed them if they pass.

If those tests fail, I fix them and/or refactor them as necessary
and move them to a subdirectory as appropriate. If it is not immediately
apparent what they are testing, I skip them.
Tests deleting an object from a ForeignField
Renames "1:n" tests to "n:n"
Adds (failing) tests for modifying backrefs on the remote object
Renames 1:1 tests to 1:n
warn on storedobject __setattr__ if key not in fields and not special
Adds tests for assignment to and deletion from 1:n ForeignFields
Adds test for assignment by index of a ForeignList
Adds test for the "in" operator on ForeignList fields
Adds a (failing) test for inserting a new object into a ForeignList field.
Adds (failing) test for removing an object from a ForeignList via the .remove() method.