Continuation of @dbushong's work on #2
Improved and rectified some error messages
Added Date, NaN, null, undefined, fixed docs and implementation

Fixes #2
Improved stringify output to do NaN right
Use _'s less false-positive _.isNaN
Also consider Invalid Date date objects non-Dates
Merge pull request #6 from groupon/hasType

Implemented hasType and notHasType