Minor changes to hashing and cache aging algorithms.

Hashing was changed slightly to exploit the fact that a significant amount of real world JSON strings / keys will have a high percentage of ASCII characters.

Cache aging was modified to use an AIMD (additive increase, multiplicative decrease) policy.  When an item is found in the cache, its age is incremented by one using saturating arithmetic.  Ages are "quasi-randomly" aged using unsigned right shifts, or in other words its age is divided in half. Since ages decrease far more quickly than they increase, the cache can quickly adapt and converge on the "hot set".
Silences some warnings that newer versions of the compiler(s) complain about.

1. Some of the string format argument specifiers used `%lu` to display `NSUInteger` values.

`%lu` specifies a type of `unsigned long`, and `NSUInteger` maps to either `unsigned int` or `unsigned long`, depending on whether or not you're targeting a 32-bit or 64-bit ABI, respectively.

On a 32-bit architecture (at least for the architectures / compilers that JSONKit will be used on), `unsigned long` and `unsigned int` just so happen to use the same primitive type- a 4 byte 32-bit value.

On a 64-bit architecture (again, for the architectures / compilers that JSONKit will be used on), `unsigned int` stays the same at 4 bytes, however `unsigned long` grows to 64-bits / 8 bytes.

The long and the short of it is this that using `%lu` allows you to easily support 32-bit and 64-bits with out a lot of effort.

However, technically, the C standard says that `int` and `long` are strictly different ranks, and it's generally considered poor form to take advantage of platform specific details in this case.

The proper fix is to add explicit type casts to `unsigned long`.  This makes the implied behavior explicit, and eliminates the warnings from newer versions of the compiler.

FYI, `gcc` did this for a long time and seems to be something that is either new or enabled by default in `clang` / `Xcode`.

Other good warning flags you should consider enabling by default (from Xcode 4.3):

Implicit Conversion to 32 Bit Type
Suspicious Implicit Conversions
Hidden Local Variables

2. Newer compiler warns about deprecated `id` `isa` usage.

I found jwz's commentary about breaking API's pretty funny: http://www.jwz.org/blog/2012/06/i-have-ported-xscreensaver-to-the-iphone/
Add a `#ifdef` to silence a `clang` static analyzer false positive warning.

The current (at the time of this writing) version of the `clang` static analyzer is complaing that the `objects` pointer is `NULL`.  This is a false positive warning from the analyzer.
Removed direct access to ObjC isa pointers to silence deprecation related compiler warnings.
Merge remote-tracking branch 'mz2/master'