OSGi support for the leshan-client

[jvermillard]

looks good to me, can @sophokles73 or @ingoschaal review pom.xml OSGI exports?

[mcarrer]

I made a couple of changes based on the feedback:

• Fixed copyright headers in pom files.
• Added leshan.util in the leshan-core exports
• Changed the packing of leshan-client not to embed leshan-core. Now the following bundles are required to run the client: slf4j-api, protobuf-java, leshan-client, leshan-core, gson, californium-osgi. It's few more bundles but it encourages modularity and re-usability.

[mcarrer]

FYI, here is a link to of an early Kura bundle using leshan client: https://github.com/eclipse/kura/tree/lwm2m/kura/org.eclipse.kura.lwm2m

[jvermillard]

rebased & merged, thx!