Added replies proto
Add Error message to replies
Add a step field to reply data interval.

If the requested data interval is not available in the database, the server is
now able to add additional info.

Example:
1. The client requests a statistic with interval 'hourly'
2. The server knows the data is only saved in 4-hour steps, so it adds the
'data_interval_step' field with value '4'.
Remove redundant fields in replies, add doc

The address and data_interval fields in the reply are redundant - the client
knows these values.
Merge pull request #1 from cooox/master

Add the replies protobuf files