Allow searching for toll-free numbers

Fix numbers()->purchase() when providing a string

To purchase a number you must provide a number and the associated
country. We previously tried to automate this using `searchAvailable`
but this only searches *owned* numbers.

To purchase without using a number object, you can now call
`numbers()->purchase($number, $country);`

Resolves Vonage#100

Fix Conversation::Create() response (id, not uuid)

Allow JWT generation when using the Container credential type

Expose JWT generation to the public

Add support for fetching a list of conversations for a specific user

Use keypair auth for Stitch endpoints if a container is provided

add `getCost()` to `Number`

Bump version

Feature/user agent (Vonage#89)

* change 'user-agent' to 'User-Agent'
* update version number from '1.0.0' to '1.1.3'