Prevent field collisions

[jacobwegner]

Currently, formly uses Field.name to construct dynamic fields on PageForm (and also to add field targets).

If a form has two identically named fields (or two identically named follow-on fields), all sorts of problems come up:

• only the last-instantiated field is displayed to the user
• only the last-instantiated follow on is displayed to the user
• the POSTED data may be validated or saved to the wrong field

I noticed this while investigating #16