Unmovable blocks: make gray, don't highlight on click, handle dragging movable block above unmovable blocks

*NOTE: This is just for review / comments. We <b>don't want to merge this</b> until we have better acceptance / unit tests for blockly-core (and maybe even until after some blockly-core refactoring).*

This set of changes:

1. Adds gray color style to immovable block

  ![image](https://f.cloud.github.com/assets/206973/2415199/3cea075e-aaf2-11e3-8b6b-fc8eb80bdd2d.png)

2. When a user starts dragging a moveable block, it brings with it all of its children above the first immovable block.

  ![image](https://f.cloud.github.com/assets/206973/2415227/925ac50c-aaf2-11e3-97fe-7b6b512ace8d.png)

  ![image](https://f.cloud.github.com/assets/206973/2415231/97991f82-aaf2-11e3-82bb-2ea985af90b2.png)

3. The first immovable block is re-attached to wherever the first movable block was.

4. Makes unmovable blocks not highlight on click