GEOMESA-432 Improvements to GeoMesa Tools script 

GEOMESA-432 This PR addresses some usability issues with the geomesa shell script. Now environment variables
for hadoop and accumulo can be optionally set, and individual paths can be overridden by the user.
Also more informative messages are printed when certain variables are not set. 
I can look into that, GEOMESA_HOME currently needs to point to the parent directory of this file (which is in GEOMESA_HOME/bin).