Do not replace html if response is empty

If the form has a file input, then the jQuery Form plugin won't handle correctly the response status and will enter in the success callback even if there is an error (see http://stackoverflow.com/questions/3995355/jquery-form-plugin-no-error-handling/7019723#7019723)

This fix checks the response length before replacing the html with a potential empty response.
Add `dataType: 'html'` to all ajax calls
add subclass parameter in jQuery request for field_dialog_form_action_ function

Need to refresh entities list (add new entity) in form after creation via popup.
Use Admin::getActiveSubclassCode method to get subclass parameter without XSS issue
sonata install instructions
Merge pull request #250 from tashanta/fixSubClasses

add "subclass" parameter in jQuery request to refresh form list after creation via popup
Merge pull request #256 from tiagojsag/docs

Installation instructions
Merge pull request #255 from tautrimas/master

Add `dataType: 'html'` to all ajax calls
Merge pull request #244 from EmmanuelVella/patch-6

Do not replace html if response is empty
Fixing notice with _per_page default parameter value
Merge pull request #258 from Bladrak/fixedPerPageNotice

Fixing notice with _per_page default parameter value
installation docs
Merge pull request #259 from tiagojsag/install_docs

installation docs
Merge branch 'sonata/master'