Added UploadImage WebMethod

Along with wrapper methods UploadDirectoryImage and UploadEventImage
Added url attribute on Dropzone.js element

Url now points to Web method instead of Form post url
Removed Image file check from code behind

as it is handled by web service now,
added GetImagePath() method which calls webservice for image upload
Parsing XML response in dropzone.js

.. and reading image upload url from Dropzone element's "data-url"
attribute