Adds a Select2TextInput widget

Adds a Select2TextInput widget which provides a combo box for
non-choice fields which can provide a list of pre-set choices, or can
accept arbitrary input.
Revert ddf885d..ac6a857

This rolls back to commit ddf885d356af64f7d1404dc7786e343005ee0d81.
Adds a Select2TextInput

Adds a Select2TextInput widget which provides a combo box for
non-choice fields which can provide a list of pre-set choices, or can
accept arbitrary input.
Minor changes

Use class var instead of instance var for INLINE_SCRIPT, also,
lowercase it.
Merge pull request #5 from mkoistinen/Select2TextInput

Select2 text input