Support Java 8 java.util.Optional type #674

It's a little bit more than adding a built-in conversion:

• For Optional<T> prop source property:
  • add check for prop.isPresent() in addition to the null-check (in every spot where we currently use our null-check assignment wrapper)
  • using prop.get() to fetch the value
• For Optional<T> prop target property:
  • use Optional.empty() instead of null for null-assignments, or Optional.ofNullable(..) for other assignments.

Optionally (pun intended) also support the equivalent in Guavas com.google.common.base.Optional, which uses different names for the factory methods.