Package riemann-java-client as Osgi bundle.

In order to use the riemann-java-client in an Osgi environment (like Eclipse Virgo), the riemann-java-client should be packaged as an osgi module.

There is a maven plugin from apache felix to package Jar as Osgi bundles and so I accordingly changed the pom.xml file to package the java client as a bundle.

The only change to the jar artifact is the MANIFEST.FM file, that will contains osgi information.

I made the imports for com.codahale.metrics and com.yammer.metrics optional, as it seems that they are not required for simple use of the client. Is it correct?