Coexistence with old forms library

Yesterday I made this minor changes to new forms library. Now it can coexists with old forms library - to ease transition from old to new library. Also, I fixed errors when there is no old forms library at all. 

Now, if we add this library to our projects that use old forms lib, everything will still work as before. Transition is painless: 

- when we put our forms into {{#form}} ... {{/form}} blocks, everything will still work as before but will use old forms code.

- then we can change from old to new code (`formHandleSubmit` etc.)

- and then we can remove old forms library.
 
@perak You can actually remove `Forms.handleSubmit` altogether, I decided we won't use it in any new code.