[OSX] Use different data directory based on version

Currently the OSX App uses the following path for data:

datadir="$HOME/Library/Application Support/Couchbase"

Instead it would be nice if it used something based on the current version, e.g.

datadir="$HOME/Library/Application Support/Couchbase450"

This would resolve upgrade issues (Although potentially inefficient with space) and would also make it easier to keep multiple versions kicking around for testing. This is the approach that I've found other OSX applications use to facilitate upgrades.