Added ability to have multiple data-dimensions

Fixes issue #11
Allow out of page slots to be created

Related to issue #9
Updated callbacks
Added defineOutOfPageSlot to the dfpBlocked function
add check so gpt.js is only loaded once

This fixes an issue where repeated calls to .dfp would result in
multiple copies of the doubleclick javascript.

This doesn't just keep track of its own state because it's possible for
gpt.js to get injected some external means.

Uses a kinda crappy selector, `src*=` because it could be http... it
could be https... and there could be GET parameters. But it's only run
once. After the first time, dfpIsLoaded becomes truthy and the selector
isn't evaluated again.

The script blocking detection appears to still work.
Fixed merge conflict
Merge branch 'you-only-load-once' of https://github.com/crccheck/jquery.dfp.js into crccheck-you-only-load-once
Fixed JSHint errors
Readme tweaks, version bump
Fixed errors with dimensions, re minified
Added contributors
More updates to the readme
Fixed merge conflict and more readme edits
Merge pull request #15 from coop182/update-documentation

Update Documentation