Initial go at an NTLM Client that will do session signing/sealing

There is some duplication between `Net::NTLM` and `Net::NTLM::Client::Session` but it was necessary to decouple some of the processing without effecting how `Net::NTLM` is being used for legacy purposes. Maybe we can rectify some of this in a 2.0 release.

CC: @pmorton This is required in order to get _Negotiate_ session encryption working for **WinRM**.
rspec has a neat feature that lets you replace this with
```ruby
expect(t2).to have_flag(:UNICODE)
```
which i think is nicer to read.

Also, `t2` seems misleading as this is a `Type1` object; it should probably be `t1`