Features/message signing

This is a two parter.  The message signing piece which fits in pretty cleanly and the consumption of signed message to dynamically control Heka sandbox filters.  The dynamic control piece alters the original static configuration design assumptions.  I had to create three helper functions to access the configuration and a method to get at the FilterRunner message matcher.  There is also some duplication of configuration and plugin startup code (createRunner) since the process is slightly different on a running system.  The dynamic configuration supports start/stop and data (sandbox state) is intentionally not preserved.  Since messages can be missed during the start/stop cycle there could be a gap in the data causing confusion.  The ability to replace a running sandbox with no message loss should be possible but at the current time would require changing even more apis.