ReplicaSet support

Solves bug #493 

**NOT MERGE** before:

*  /etc/init.d/contextBroker has been fixed to take into account the new parameter BROKER_DATABASE_RPLSET
* Install & Admin manual describes the usage of new `-rplSet` CLI option and BROKER_DATABASE_RPLSET parameter
This concatenation technique could be used to simplify the above ugly if/then/else construct in lines 68-84 with:

```
# Mandatory parametesr
BROKER_OPTS="-port ${BROKER_PORT} \
   -logDir ${BROKER_LOG_DIR} \
   -pidpath ${BROKER_PID_FILE} \
   -dbhost ${BROKER_DATABASE_HOST} \
   -db ${BROKER_DATABASE_NAME} \
   ${BROKER_EXTRA_OPS}"

# Optional parameters
if [[ ! -z "${BROKER_DATABASE_USER}" ]]; then
   BROKER_OPTS="${BROKER_OPTS} -dbuser ${BROKER_DATABASE_USER}"
fi
if [[ ! -z "${BROKER_DATABASE_PASSWORD}" ]]; then
   BROKER_OPTS="${BROKER_OPTS} -dbpwd ${BROKER_DATABASE_PASSWORD}"
fi
```