GEOMESA-44 Use SimpleFeature rather than an STIndexEntry

Refactored index to use SimpleFeature throughout rather than
requiring developers to extend SpatioTemporalIndexEntry.  Also,
the passed in SimpleFeatureType no longer needs special fields
for the geom index or dtg indexes.  By default, GeoMesa will use
the default geometry of the SFT and users can specify which field
to use as the dtg field.
GEOMESA-45 Users should be able to initialize and ingest features from the Accumulo shell

Implemented initialize, ingest, and list features commands for use
from the Accumulo shell.