WPSC_Meta Class Implementation

WPSC_Meta class implements meta value access with efficient caching for
small and large values.  The class provides methods for accessing meta
data items individually and as collections.
Merge pull request #434 from JeffSparkleGear/WPSC_Meta-Class

Initial pass at WPSC_Meta Class Implementation.  Should be considered very, very beta.  Test with a hammer, especially for the initial load - where a site may upgrade and users are already actively a part of a session.  That's a situation I'm a bit concerned about at the moment.