Finish basic preupload of slides using a url

Accepts a single url with content to preupload.

refs #539
Pre-upload of slides only using URLs, allow multiple values

Accept multiple URLs and uses them all when send 'create' requests via POST.
Removed the components to upload slides via content set in an input. Doesn't
make much sense to set the entire content of an image or pdf in an input.

refs #539
Update bigbluebutton-api-js to mconf/bigbluebutton-api-js@64c522f10348e
Add new parameters supported by BigBlueButton 0.81
Update the link to Mconf-Live's API docs
Slightly change in the thanks message in the header
Improve the padding method to make it more generic and consistent
Remove compiled files from master, keep them only on gh-pages

gh-pages will store the compiled files for the latest version.