Support gwt:run-codeserver out-of-the-box

Ideally, we'd remove the user.agent=safari from the _dev.gwt.xml, but it's
still useful to speedup compilation in -Pdraft.
Fix typo in README
Repurpose draft profile for superdevmode
Update dependencies and plugins (except GWT and servlet-api)

Don't update to Guice 4.0-beta as it's incompatible with GIN 2.1.2
(problem with the shaded Guava classes, no longer shaded in Guice 4.0)
Update to GWT 2.6.0

Update servlet-api to 3.0.1 (compatible with GWT 2.6.0 DevMode), and
switch Java 7.
Update user.agent to remove ie9 (replaced with ie10) and
opera (without replacement).
Add -Dgwt.codeserver.port for GWT-RPC with SuperDevMode
Remove _dev modules now that they're empty.
Add SuperDevMode section to README
Remove draft profile