Shortcut to default account resources through client delegate methods

There was an awful lot of repetition in using the REST client with the default account (`client.account`). To overcome this, once the account is set up in the client initializer, this pull request then sets up delegate methods from the client to the available resources in the account. So, `client.account.messages` becomes `client.messages` and so on.

There are a lot of documentation changes in this request, but the important addition of code can be seen [here in client.rb](https://github.com/philnash/twilio-ruby/blob/cfc15a913b98603ba49ecf6f08761c12304c9387/lib/twilio-ruby/rest/client.rb#L214-L229).

What do you think?