Add some of the metrics mentioned in #718

use single static initialization of available metrics monitor registries

* This changes the original implementation to work in a similar way to
how slf4j selects and loads an implementation.
* Uses a single static instance so intialization is done only once.
* Doesn't throw IllegalStateException if multiple implementations are
found on the classpath. It instead selects and uses the first
implementation returned by iterator()
* Class left as an iterable to keep the API the same

add yammer metrics to examples to allow them to publish metrics

publish the number of threads used in an EventLoopGroup see issue #718

* seems like the better place to put this because it sets the default
thread count if the MultithreadEventLoopGroup uses super(0,...)
* It also happens to be the common parent class amongst all the
MultiThreadedEventLoopGroup implementations
* Count is reported for
io.netty.channel.{*,.local,.socket.aio,.socket.nio}

fix cosmetic issues pointed out in pull request and updated notice.txt

see https://github.com/netty/netty/pull/780

count # of channels registered in single threaded event loop

measure how many times Selector.select return before SELECT_TIME