Confirm that we can store emoji

and as a result, other extra wide unicode.

This was expected to work here. It is expected to fail with
mysql. See

  https://github.com/TiddlySpace/tiddlyspace/issues/1033#issuecomment-15951978
Remove 'value' from the primary key of field table

It is not needed and without it we can have a longer value
in mysql. Fixes #4 (which has more details on the problem).