Makes rubyntlm dependency optional

The following validations will be done if the request object is using NTLM authentication:
* Sends a friendly message if user doesn't require 'httpi/auth/ntlm'.
* If the user doesn't have rubyntlm installed, HTTPI raises another friendly message asking user to install it.

Main idea behind this PR:
* Who wants to use HTTPI and don't need NTLM support, don't need to install and require rubyntlm.
* Who needs NTLM, just install rubyntlm and require 'httpi/auth/ntlm'.