Register in an Application class instead of "first Activity"

As mentioned earlier today in issue #3 it would be better practice to register the notifier before any services and/or activities are started to catch all errors. This pull request does update the Readme.md and example App to do exactly this.

Note that this also makes registering more robust for apps with Activities started by Intents from outside and apps with multiple services etc. These are cases where there is no real "first activity" and the app could crash before Bugsnag is registered.

Hope you can pull this in, if anything has to be fixed before a pull is possible please let me know.

Ps. I think that this should also be merged into the V2 branch, shouldn't be to hard at first glance. But please let me know if I should make a separate pull request for that.