Make $dict() work correctly with ARC

The _dictpair struct didn't work properly if one of the parameters to $dict() was a temporary
value -- it'd be released before the call, causing the pointer to be garbage. Fixed by using
an array for _dictpair instead of a struct.
Fix/improve exception -my_callStack method
Fix string constant comparison warning in Logging.m

Fixes #2
Fix unnecessary 401 challenges

MYURLUtils' method -my_credentialForRealm: is returning a credential with no persistence
when the URL has a baked-in username and password. This turns out to prevent NSURLConnection
from proactively sending the same credential on the next connection to the host, with the bad
effect that every connection initially gets a 401 and only then gets the credential and retries. This
slows down performance.
Fixed this by changing the persistence to NSURLCredentialPersistenceForSession.
I believe this fixes #197 in TouchDB.
Less logging from MYDynamicObject.
Added -[NSURL my_URLByRemovingUser]
Just fixed project name in source comments.
Slight improvement to logging

Log() now writes a double-bar instead of a "|" after the timestamp if
the message is from a background thread.
Added -[NSURL my_sanitizedString], for logging URLs without passwords
Fixed compiler format-string warning in Target.m
Added MYDeferDealloc()

ARC-friendly equivalent of [[obj retain] autorelease].