Increasing maximum number of form fields

Hi,

There is a little bottleneck in the application, when you add a big number of fields to one form, you could easily exceed PHP's POST parameters limit (max_input_vars: default 1000). 

To fix the problem i made a modification where instead of sendings fields as POST request, i serialized them in one post field in json, it can handle illimited number of forms until POST max size is reached.

There is still some performance issue, though, with a big number of fields, the ajax request takes ages to process, i will look into it later.
Add a $this = $(this), for better performance