Make it easier for users to construct and use a security token

In comments on ACCUMULO-1041, we discussed options for simplifying the API for "logging in" or, more appropriately, getting tokens to create a Connector with.

The main idea is:

1. add "Connector getConnector(String p, Props)" AND "SecurityToken getSecurityToken(Props)" to Instance
2. would be nice to have a way to automatically get possible prop keys

The idea of properties here is to provide an interface for passing any set of credentials to an authentication system to either get a connector directly (with the token being constructed internally), or to get a (potentially) re-usable token. This is especially important for components like the proxy, for which we'd rather not implement every possible security token type, nor do we want to require every possible authentication service be written to support every language that the proxy supports.