preparePlainTextValue implementation
Relationship is required

This add the data-required attribute in order to
remove the 'none' option. This fix requires a core change,
which is https://github.com/symphonycms/symphony-2/pull/2114
Merge pull request #66 from DeuxHuitHuit/plain-text

preparePlainTextValue implementation
Merge pull request #68 from DeuxHuitHuit/data-required

Relationship is required
Add checkPostFieldData function so that the required values can be detected by events
Stop storing the association on the field
Fix required check