[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 an interface for getting and setting the redstone output level of a command block via the BlockState interface. This can be be accessed inside commands via ```BlockCommandSender.getBlock().getState()```

**PR Breakdown:**
This PR adds two new methods to CommandBlock, 

    void setRedstoneLevel(int level) 
    int getRedstoneLevel()

The related CB PR implements these.

**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):**
To be updated once other PR is written and url obtained

**JIRA Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-3833