Improvements to form submissions (upload field, custom notification email)

Here are some improvements to form submissions, that include:
1. new field type: "upload", that allows to upload single file along with submitted form (has customizable file extension and file size limits)
2. ability to specify separate notification email for each form being created (for now all emails are sent to admin)

Also some minor bugfixes:
1. when you specify absolute url to form thank you template, e.g. http://www.site.com/thank_you.html, then it won't work (because of all data from front-end is escaped resulting broken url)