Revert "Bumped v0.6.2"

This reverts commit c9152b4cef5131466ff7aa72523bf1bd7ead416d.

@nqd - According to
https://github.com/mcollina/node-coap/blob/master/CONTRIBUTING.md
the original author is still responsible to make formal releases,
even if you have commit access.
I am reverting this commit, and untagging.

The new blockwise implementation is going to be released
as 0.7.0, following semver: http://semver.org/.
Updated README with the coap blockwise implementation.
Bumped v0.7.0.
Added gitter badge.
when server.listen() has no argument, server still works