[B+C] Add getter/setter for command block redstone level, adds BUKKIT-3833

**The issue:**
Currently there is no way for a plugin (either via event or command) to set or get the redstone output level of a command block.

**Justification of this PR:**
This PR implements the code for getting and setting the redstone output level of a command block via the BlockState interface. 

**PR Breakdown:**
This PR implements the new methods for CommandBlock from the relevant Bukkit PR.

It also modifies the behaviour of TileEntityCommand. A flag has been added that is set when the implementation of CommandBlock updates the redstone level of the command block.

If this flag is set between resetting it at the start of command processing and the return
result, then the block state has been changed by a command executed by the command block, and thus the method will return the current level of the command block instead of one equal to the number of successfully completed commands). This allows the block state to be updated inside a command.


**Testing Material and Results:**
A sample plugin code can be found here: https://gist.github.com/tehbeard/5208456 
This implements a command that when executed by a command block will take the first parameter, convert it to an int and then set the command block's redstone output to that value.

**Relevant PR(s):**
Bukkit/Bukkit#809

**JIRA Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-3833