Added possilility to use pool of SandCastles.

Pool-object is a drop-in-replacement for SandCastle-object. It runs
scripts on a pool of SandCastle-instances.

Only one script is allowed to run at a time on a single instance. This
is enforced by a queue system.
Fixed test.
Fixed 'pool should run scripts on non blocking instances'-test timeout.
Minor cleanup.
Added missing semicolon.
Review fixes. And changed API: 'shutdown' is now 'kill'.

Fixes according to bcoe's comments.

Changed shutdown to kill to be 1-1 compatible drop-in-replacement for
SancCastle-object.
Merge branch 'feature-pool' of https://github.com/ndob/sandcastle into ndob-feature-pool

Conflicts:
	lib/sandcastle.js
	test/sandcastle-test.js