Added initial CachedReferenceField
fixes for rebase branch
added CachedReferenceField restriction to use in EmbeddedDocument
refs #709, added CachedReferenceField.sync_all to sync all documents on demand
refs #709, fix typos
refs #709, added support to disable auto_sync
fixes for python2.6
added documentation about CachedReferenceField
small fixes for python2.6
Merge pull request #709 from wpjunior/cached-reference-field

CachedReferenceField implementation