Feature: Possibility to use a pool of SandCastle instances.

Pool-object runs scripts on a pool of user specified number of SandCastle-instances (equals sandbox-child processes).

Pool-object is a drop-in-replacement for SandCastle-object: If user want's to switch to pooling instead of single SandCastle, only the initialization of SandCastle/Pool-object has to be changed. See tests for additional examples.

Only one script is allowed to run at a time on a single instance (enforced by a queue system). This basically prevents single bad-acting script choking the whole child-process and when killed taking other scripts down with it.