Modify the DurationPropertyDefinition and DurationUnit classes so that they have more user-friendly encode/decode methods:

* the DurationPropertyDefinition encoder is unchanged: it uses the base property's base unit
* the DurationPropertyDefinition decoder now supports a mixed format in addition to the old format: now users can specify durations using a mixture of units such as 1h30m instead of 90m (both are permitted)
* the DurationUnit class has a toString(long) method which applications can use to get the above mixed unit representation (e.g. CLI)

The unit tests are updated. During the precommit the password policy test cases failed because they were testing some invalid durations which were not being checked in the DurationPropertyDefinition test suite. I've put these missing test cases in the DurationPropertyDefinition test suite. In addition, I've numbered the PWP test data entries so that they are easier to debug in future.