spec file update to CentOS 6

- CentOS 6 support
Remove rpmbuild warning on BuildPrereq now deprecated for BuildRequires
- opensips 1.9.0 support
Updated list of excluded modules
Updated list of file in rpm of core and modules
- Some new rpm optional modules are added:
db_http pi_http json cachedb_couchbase cachedb_memcached   cachedb_redis event_rabbitmq db_berkeley perl db_perlvdb carrierroute mmgeoip ldap h350 identity regex dialplan Xlog mi_xmlrpc db_oracle
Merge pull request #5 from fabriziopicconi/patch-3

spec file update to CentOS 6