dashboard: Remove GC spaces list print

When using the dashboard command, a list of gc pools is printed to screen
before the main table layout is shown.

This is caused because of `(jvm.pools | { name -> .. })()`. The closure, in
this case, returns the name of the pool in question. This causes it to be
printed to screen while the UIBuilder.table() is generated.

Explicitly placing "null" at the end of the closure makes Groovy consider this
as the return value, thus nothing gets printed to screen before the dashboard
is displayed.
CRASH-163 : Web connector
Check that resource URL returned by classloader is not null before processing it
Minor issue : should use the correct DOM element in CRaSH.js
Allow to obtain the related session id when one exists
Allow to customize the created PluginContext
Implement a CRASHID cookie that can be useful for tracking things accross new websocket conversations
Better logging in crash.js
Minor logging improvement
Make egrep behave correctly when no pattern is provided
Minor improves
[maven-release-plugin] prepare release 1.3.0-beta13
[maven-release-plugin] prepare for next development iteration