Add returntypes where possible

[Zombaya]

As a replacement for #40, I have created this PR to add return types wherever it was possible.

This should add the return types as added by the interface as well.

For the ones of the interface where it was not possible (because mixed doesn't exist yet on 7.1), we add it with in a docblock with a ReturnTypeWillChange-attribute.

Additionally, I changed some instances of "static public function foo()" to "public static function foo()".

Finally I added "public" to all constants which were public implicitly.

Dev-dependency

I added pear/pear-core-minimal as a dev-dependency because otherwise I was not able to run the unit-tests, although this will probably be fixed in pear/OLE#33.