added Dropbox logo
disabled autocomplete and added required in all fields, tweaked the code to work better with the javascript, added Dropbox logo, corrected and repositioned the header of the form
changed and defined the final visual for the form, added a responsive design through max-width (was not necessary the use of media queries), changed all values to em units, tweaked most margins and paddings
jquery rewritten, added sliding effect while hiding and showing the sign-up part of the form, added verification of the form based on what radio option was selected