Fixing a typo in option name in docs

Doc mentions "--ec2_instance_type" for command line invocation, but the option is actually "--ec-instance-type" (hyphen, not underscore) for command line.
Merge pull request #854 from darthvadar/patch-1

Fixing a typo in option name in docs
Showing progress % only when there are running jobs in jobtracker UI
Give better error message if keyfile is missing
Fixed virtualenv sample script

virtualenv should be created if the venv directory *does not* exist (now it did the opposite -- created it if it existed!).
Merge pull request #862 from santtu/patch-3

Fixed virtualenv sample script
Merge pull request #858 from sketchytechky/improve_nokeyfile_error_msg

Give better error message if keyfile is missing
Merge remote-tracking branch 'upstream/master'