Check for empty table before loading FluoFileOutputFormat file

[mikewalch]

A file created by FluoFileOutputFormat has timestamps of zero. This could cause issues if the file was loaded into a non-empty table. A sanity check could be added to stop this. Discovered while reviewing #35

[keith-turner]

This could be tricky. The map reduce job producing the rfiles does not need to know anything about the Accismus instance where files will eventually be loaded. Would need to require that information so that this check could be done.

[keith-turner]

One possible way to do this is to add a Fluo API call for loading files into a table. This call would do the sanity check and then make the Accumulo call to bulk load files. Could encourage users to use this API call instead of going to Accumulo directly.