v0.5.0 - Better validation errors, new extras, Union property tweaks

Backwards-Incompatible Changes

• Union property deserialization has been improved more reliably reconstruct correct instances (see #228)
  • This requires new strict_instances attribute on Union properties
  • It also adds strict and assert_valid on deserialize, instead of `verbose.
  • There is no guarantee that Union properties behave identically to their previous behaviour.
• Task classes are moved from properties.task.Task to properties.extras.Task

Major Features

• New ValidationError is raised when validation fails (see #224)
  • This is a subclass of ValueError
  • Multiple validation errors are now collected and raised together in one error description.
  • There is also an error hook called on validation error to customize error behaviour
• New extras classes:
  • HasUID and Pointer (see #234)
  • Singleton (see #227)