Support Java 8 Optional

[whiskeysierra]

Right now there is explicit support for Guava's Optional (see here. It would be nice to support the Java 8 version as well. Hardcoding it just like the other is probably a bad idea as it would require to drop Java 7 support, so we most likely need some plugin mechanisms there, e.g. ServiceLoader?

[zd0]

+1 for Java 8's Optional support. Personally, doesn't matter if it would be hardcoded and Java 7 is dropped.
Currently, the code that has switched to Java 8 Optional and still uses this library has ugly transformations scattered around.

[valgog]

@zd0 does this imply that we are loosing Java 7 compatibility with this change?

[whiskeysierra]

No, it shouldn't