Add new XMLString field type

Merge pull request #357 from edx/cdyer/xml-string-field

Add XMLString field type which validates input.

xblock-0.4.7: Merge pull request #339 from edx/mattdrayer/increment-release-version

Includes latest changes to XBlock core project

xblock-0.4.6: Merge pull request #335 from edx/asadiqbal08/WL-376

Latest enhancements to XBlock core

Cypress RC2

Merge pull request #301 from edx/adam/add_force_save_fields_2

Adam/add force save fields 2