Fix #1025: Turn the sbt plugin into an AutoPlugin.

Note that we cannot use the AutoPlugin in our build itself,
because we need the *abstract* set of settings. However, the
AutoPlugin is tested in sbt-plugin-test.
Merge pull request #1303 from sjrd/autoplugin

Fix #1025: Turn the sbt plugin into an AutoPlugin.
Throw more aggressively on PhantomJSCom failure
Fix initialization race in PhantomJSEnv
Additional tests for ComJSEnvs
Fix #1304: Fragment messages to limit total size to 32KB
Merge pull request #1305 from gzm0/phantom-com-fixes

Phantom com fixes