implementing locking rather than thread confinement
NSMutableDictionary was being used to confine all the non-thread safe objects, but NSMD is non-thread-safe itself...oops. Locking has been implemented. Works very fast and is much safer.
Add Class methods for constructing NSDate object based on start or end of yesterday, today and tomorrow.
Merge pull request #19 from jeddison/issues/18

Add further convenience class methods for constructing NSDate objects