Block hitboxes are generally too big

Summary:
Various blocks have hitboxes which are too large, which means that they can prohibit access to other blocks.

Steps to Reproduce:

1. Punch a single hole in the ground
2. Place a chest in the hole
3. Place a standing sign or a fence gate over the chest
4. Try and open the chest

Observed Results:
The chest cannot be accessed because the hitbox for the sign and/or the fence gate it that of a full sized block.

Expected Results:
You hitboxes should be smaller for certain blocks, listed below.

Screenshots/Videos attached: Yes (please see attached test world too)

Notes:
These blocks have hitboxes which are too large, and have been compared to the Java version:
Updated for 1.5.0.1 beta:

• Piston Arm

These blocks have now had their hitboxes fixed:
- Standing Sign
- Anvil
- Fence Gate
- Brewing stand (top part only)
- Single Glass Pane
- Single Iron Bar
- Fully grown crops (eg Carrots)

Please see attached test world for working examples of these

nb. The following blocks also have hitboxes which I consider too large, but may be working as intended as they have the same hitboxes as the latest Java version: Hopper, string, cauldron, bed, cobweb.