Adds a block configure syntax to the Rest client.

This makes instantiation of multiple clients with the same credentials easier.
For example:

    Twilio::REST::Client.configure do |config|
      config.account_sid = 'someSid'
      config.auth_token = 'someToken'
    end

    client = Twilio::REST::Client.new
    client.account_sid
    # => 'someSid'
Updates the config block to work across more classes.

It didn't make sense for the Twilio::REST::Client to look after configuration, as both the Twilio::Util::Capability and the Twilio::Util::RequestValidator both use one or both of the credentials too. So, I moved the configure method to the Twilio module directly, the Configuration class to the util/ directory and updated the classes that take account_sid and auth_token as an initializer to use the config if it is available.
Updates docs for creating clients and capabilities.
Delegates Twilio#account_sid and #auth_token to configuration below
Merge pull request #105 from philnash/configure-block

Adds a block configure syntax