doc updates and bug fixes

Mostly this is document updates.  You can see the new docs here:
http://hildrum.github.io/streamsx.hbase/

Fixes  IBMStreams/streamsx.hbase #14 (doc updates),
Fixes  IBMStreams/streamsx.hbase #9  (context checker),
Fixes  IBMStreams/streamsx.hbase #11 (String encoding)