Implement std::error::Error for AsyncError
Implement std::error::Error for AsyncError

Fixes #22

benaryorg/eventual,tempbottle/eventual,seanmonstar/eventual,carllerche/eventual,mlalic/eventual,carllerche/eventual,seanmonstar/eventual,mlalic/eventual,blaenk/eventual,sisirkoppaka/eventual,blaenk/eventual,sisirkoppaka/eventual,benaryorg/eventual,posix4e/eventual,tempbottle/eventual,posix4e/eventual