Moved configuration to IBackend

Changed all the backends to control their own config loading so that we
can isolate a backend's code.
Added MEF support for backends

The available backends are now loaded via MEF.  We look for backends in
the current assembly and in any DLLs that are in the same directory as
the EXE.  We could change this to look in a Plugins directory if we
thought that would be cleaner.  I went with a single directory for
simplicity.

Log4net is now initialized before the Statd instance is created so that
we can log configuration details before the statsd instance is created.
Added Example External Backend assembly

Created a File Backend that simply appends lines to a configured text
file.  This is just a proof of concept that backends can be defined and
loaded from an external dll.  The project currently references a new
project - statsd.net.core.  This assembly has just the basics needed for
a backend - the IBackend interface and a few more classes tied to it.
The statsd.net.shared assembly references a bunch of other projects that
we wouldn't want to be required for every backend.

The example backend does not currently add itself to the main bin
directory.  You'll have to do that manually.  This could be added if we
thought it was helpful.
Merge pull request #20 from joshclark/plugable-backends

Add support for plugable backends - thanks @joshclark!