Add install build target to install the formatter

This publishes the formatter and installs the 'ceylon format' plugin
script. See ceylon/ceylon-compiler#1776.

(The XML is mostly copied from @quintesse here:
http://transcripts.jboss.org/channel/irc.freenode.org/%23ceylonlang/2014/%23ceylonlang.2014-08-31.log.html#t2014-08-31T21:50:07 )