bin/elasticsearch: add help, fix endless loop

This change adds command line help for all options to the es start script.
Both `-h` and `--help` options are accepted.

Also, an endless busy loop in the long options parser was fixed: running the script with a long opt parameter w/o value (e.g. `elasticsearch --buuuurrrnn`) the long option parser would end up in an endless busy loop.

Fixes #2168 
cant we just call `shift ; shift` (which will throw an error as expected if there are no more arguments) instead of `shift 2` and the whole if-statement thing?