Add update and delete submission methods to API

[benkeen]

See: formtools/core#160

[benkeen]

N.B. Adding it in is pretty trivial but we'll need to update the Submission Pre Parser module at the same time.

[uprjfc]

Could update and delete have a generic where statement rather than just ID.
Or a function to find an ID using a generic where statement.

[benkeen]

Yeah, that'd be handy too. Thanks @uprjfc - good idea.