GUAC-340: Add support for "username" parameter type.
Merge branch 'master' into GUAC-546-REWRITE

Conflicts:
	guacamole/src/main/java/org/glyptodon/guacamole/net/basic/rest/protocol/ProtocolRetrievalService.java
	guacamole/src/main/webapp/scripts/admin-ui.js
	guacamole/src/main/webapp/scripts/service.js