- Changed 'transform' method to use ValueInfo throughout. This fixes a number of obscure bugs
- Changed the 'add' and 'sub' methods to default to "now" rather than null. This means null passes through rather than being interpreted as the current time, to be consistent with other date tools
- Also changed the 'add' and 'sub' methods so that they operate correctly with invalid date strings
- Added tests for the class; improves #66
- Modified TTRSS tests because the "iso8601" format string in ValueInfo is different from Date's older format
Also move date formats to the ValueInfo class
Standardizing on immutables avoids any possible ambiguity in the API of the resultant value, as well as any ambiguity as to whether a DateTime output instance is the same instance or a clone (they had been clones)
