Shared instance should be declared static

Hi,

Came across a clash between `DCIntrospect` and the `PayPalMPL` library; a symbol clash over `_sharedInstance`. Since PayPal's library is closed source, I can only modify `DCIntrospect`'s. In any case, the shared instance can and should be declared `static` and this prevents a clash between this and any other library.

Thanks,

J