Adding support sumbit_multi operation

[vetss]

For supporting of sumbit_multi operation I have created 2 pull requests (into master and netty4 branches): https://github.com/fizzed/cloudhopper-smpp/pull/3 https://github.com/fizzed/cloudhopper-smpp/pull/4

Can you please check them and merge into the repo code.

PS: The pull request https://github.com/fizzed/cloudhopper-smpp/pull/4 shows me some error in unit tests. I have tried to reproduce the error locally several times but all tests pass at my side. Can we retest it ones more ?