Change net/ntlm check to be at runtime
Change net/ntlm check to be at runtime

Not everyone likes their errors to occur at interpretation time. I'm not a huge
fan of runtime errors, but it seems like the ruby community is. At the very
least, this change DOES make it possible to quickly choose a different adapter.

This should help address #127

liftforward/httpi,savonrb/httpi,sobrinho/httpi,mbrung/httpi