Use "complete" path for custom html/js elements, not based on project dir

Conflicts:
	bin/rock-webapp
	lib/rock/webapp.rb
check whether the custom dir is set before mapping it
added frame display .js
check whether the custom dir is set before mapping it
use Rack::Directory instead of Rack::File, allows an index of existing ui elemments
Merge pull request #5 from planthaber/custom_ui

Adds a PATH variable to custom UI Elements which are presented through the webapp