make appbundle target for OSX development

Adds an 'appbundle' target that creates a top-level Bitcoin-Qt.app/
Modifies the 'deploy' target so that it leaves behind Bitcoin-Qt.app;
make clean will remove it.

This is very useful in combination with http://www.rubicode.com/Software/RCDefaultApp/ for testing payment request handling (I compile a source-tree Bitcoin-Qt.app/ and then use RCDefaultApp so that it is used to handle bitcoin: URIs).