Add a skip_connect option to avoid making a database connection on initialization

[aharpervc]

Todo

• add :skip_connect init opt
• add a test for the option
• add a test for any offline instance methods we care to validate, so at least escape
• update readme

Notes

This enables creating a TinyTds::Client instance without an actual database connection. Normally this isn't useful, except for when you want to call escape offline, such as for automated testing or preparing SQL statements outside of the context of a database.