GEOMESA-164 

adding geomesa pages to geoserver - accumulo stats, hdfs stats, hdfs config, initiate m/r jobs
It looks like the `Command` classes are designed for end-users, not programmatic use; e.g. they accept a command-line as input and output text. So to use them, we'd have to configure an instance of the shell, send user input as strings, and then parse the output as strings. Should I go with that approach? It will still be somewhat brittle, given the input/output parsing.