[snmp/agent] Added some description and the behaviour attribute
[snmp/agent] Fixed snmpa_mib_data behaviour and related

Finalized snmpa_mib_data behaviour. Updated mib-server and
tttn module accordingly. Also assigned proper version, updated
app and appup files.
[snmp] Added doc/index.html file to git ignore
[snmp/agent] Updated documentation and release notes
[snmp/agent] Fixed some test cases