add Geotrigger API client

start of a ruby geotrigger api client, not currently required from `arcgis.rb`, but can be req'ed once gem is installed (gem is not published?)

i guess there's some work to do on the arcgis online side?  see AGO::Session for token handling via oauth2 client creds.

all specs pass against currently deployed geotrigger api.

/c @davy @poeks @aaronpk @JerrySievert