Client IntegerLwM2mExchange fix

Hello Julien,
this is a fix for the IntegerLwM2mExchange in the Leshan client.
The main idea is to remove the convertion to string and back again for Integer type transport -
instead, transport integers according to LWM2M spec (Appendix C. Data Types), TLV Integer format. 
Makes the client also compatible with Wakaama (important for us) and the payload will be smaller.

Along with the minor fixes in the pom.xml (missing plugin version, incorrect version range).