Rename and Consolidate EventLoops

I renamed and consolidated a few `EventLoops`.

Old Name | New Name
------------- | -------------
SingleThreadEventExecutor | AbstractEventExecutor
SingleThreadEventLoop | AbstractEventLoop
MultithreadEventExecutorGroup | AbstractEventExecutorGroup
MultithreadEventLoopGroup | AbstractEventLoopGroup

For the below naming to work the following classes had to be merged.
* **AbstractEventLoop** with **EmbeddedEventLoop**
* **AbstractEventExecutor** with **EmbeddedEventLoop**, **ImmediateEventExecutor** and
**GlobalEventExecutor**, **SingleThreadEventExecutor**

Here's a comparison of the inheritance trees:
**master**
![eventexecutorold](https://cloud.githubusercontent.com/assets/1388179/3902201/ac0035a6-22ba-11e4-9267-cdc5659d3199.png)

**PR**
![eventexecutor](https://cloud.githubusercontent.com/assets/1388179/3899309/7ae15554-2279-11e4-9541-94238a5f6ebd.png)

**master**
![oldeventexecutorgroup](https://cloud.githubusercontent.com/assets/1388179/3902209/b1d83258-22ba-11e4-8260-ac32fe224218.png)

**PR**
![eventexecutorgroup](https://cloud.githubusercontent.com/assets/1388179/3899312/7f9e0a74-2279-11e4-855c-7976c242d6ae.png)

Furthermore, the system property **io.netty.channel.eventLoopThreads** was renamed to **io.netty.channel.eventLoopParallelism**.

WDYT @normanmaurer @trustin?
I think we could do this in the constructor and return the same instance all the time to reduce object creation.