WEB: Changed legacy Android Market URL into Play Store one

The term Android Market (and the used url) was replaced by Google Play Store on March 2012. Some branding guidelines: http://developer.android.com/distribute/googleplay/promote/brand.html

I also contemplating on replacing the icon with https://dl.dropboxusercontent.com/u/25959245/catpl-android.png but then decided against it, since the Android robot icon better represents the used platform.