support += too

[extrawurst]

i guess you choose ~= for adding an handler because it is the natural D way of appending but i think suporting += for that too, would be nice

[Orvid]

I initially thought of using +=, but then thought about the fact that ~= is D's append operator, which is exactly what you do with multi-cast delegates. I thought about supporting += as well, but the fact that this would cause inconsistencies with the usage of the library made me decide to support only the D-style operator. You actually can use the += operator currently, you just have to allow using deprecated methods.