Fix double alloc of persistent store coordinator

ATDataManager lazily allocates a persistent
store coordinator inside a try/catch harness,
but the code path followed where no exception is
thrown repeats the allocation and assigns it to
the var, causing a leak.

To see the leak, just issue a call to
-[ATConnect setApiKey:@""] (the argument must be
a non-nil string) and monitor the results in
Instruments.
Merge pull request #68 from YellLabs/bugfix_psc_leak

Fix double alloc of persistent store coordinator