BUG-472 Initial EXI encoder/decoder implementation in Netconf

according to http://tools.ietf.org/html/draft-varga-netconf-exi-capability-02

Change-Id: I861c126a8df1b26f477aa354ed1f8dba59d651cd
Signed-off-by: Martin Bobak <mbobak@cisco.com>
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>