GEOMESA-62 Adding ability to modify and remove features from the AccumuloFeatureStore

Added the ability to modify and remove features in Accumulo.
GEOMESA-74 Change Geoserver Plugin to hide password

PR # - Base_Branch:master
GEOMESA-75 Change Geoserver Plugin to allow blank Auths param
GEOMESA-73 Add converters to AvroSimpleFeature setAttribute() for compatibility

Included:  Adding tests for converting empty strings to nulls;
adding validation and implementing setAttributes; adding test;
creating non-conversion setters for speed; adding explicit conversion.
GEOMESA-103 AvroSimpleFeature return nulls for attrs that don't exist

Making AvroSimpleFeature return nulls like SimpleFeatureImpl for attributes that don't exist in the feature
Merge branch 'master' into wps