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**

The new inheritance tree looks as follows:

![eventexecutor](https://cloud.githubusercontent.com/assets/1388179/3899309/7ae15554-2279-11e4-9541-94238a5f6ebd.png)
![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?