Update README.md

Corrected some spelling and took into account [Issue 229330](http://code.google.com/p/chromium/issues/detail?id=229330)
added usb-label-printer sample
changed default image on label-printer app
Version 0.0 of gapi-chrome-apps library

Provides the Google API javascript client 'gapi' as
appropriate for hosted websites, or if in a Chrome packaged
app implement a minimal set of functionality that is Content
Security Policy compliant and uses the chrome identity api.
add in-app-payments sample app
Merge pull request #85 from agryson/Issue229330

Update README.md
Merge pull request #102 from mkruisselbrink/iap

add in-app-payments sample app