manager: bump version to 2.0.0

AMI has received substantial updates over the past year. Not only has the
syntax been vastly improved and made consistent (which entails many event
changes), but the underlying things that those events convey have changed
substantially as well.

After some conversation in #asterisk-dev, it was agreed that this is a good
time to jump to 2.

At the same time, since ARI will most likely use semantic versioning, we
might as well use that for AMI as well. That also affords us greater meaning
for the AMI version.
........

Merged revisions 404421 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404433 65c4cc65-6c06-0410-ace0-fbb531ad65f3