Check that the callback to gapi is defined prior to sourcing.

apis.google.com/js/client.js requires the onload callback
to be defined before loading the script. This change
throws an exception of that is not the case.
Add gapi-chrome-apps-lib to README.md
Only set content-type to json when sending a body.
Copy LICENSE file to gapi-chrome-apps-lib dir.
update in-app payments client library
add minimum chrome version
Merge pull request #104 from mkruisselbrink/iap

update in-app payments client library
Link to chrome store