Rebar2

Add rebar build system.
Don't remove autotools stuff.
Use .script file to generate version from git when applicable.
Use escripts to generate exmpp_known_* files (could be used from autotools also as it depends only on erlang).
Build with all COMPAT_MODULES.

TODO:
- put version in a common place for autotools and rebar ?
- Add dependancies checking (libxml2, openssl, etc.)
- Tested only on linux (Debian). Due to port drivers, it is likely that non unix platforms won't work out of the box.