Avoid retain cycles between RACDelegateProxy and RACEventTrampoline.

Trampolines will always have a strong reference to their delegate proxy
for its lifetime, so we can safely have a non-retained back reference,
breaking the cycle.

Fixes #743.
Prefer dot-syntax over square-bracket syntax.
Merge pull request #751 from lukeredpath/bug/fix-retain-cycle-in-delegate-proxy

Avoid retain cycles between RACDelegateProxy and RACEventTrampoline