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 
`shift; shift` will silently fail (i.e. the second `shift` will when only one argument is present), no error will be reported. My rationale for the `[ ]` statement (and accompanying `echo`) was to mimic the error message reported by `getopt` (which is used to parse the short options). So you will get the same error message if you miss a mandatory value for both short and long options.