Autoload constant modules

When using the examples in the README you will receive a `uninitialized constant` error.
To reproduce:

```
$ rails c
>> require 'opentok'
>> OpenTok::SessionPropertyConstants::P2P_PREFERENCE
NameError: uninitialized constant OpenTok::SessionPropertyConstants
>> OpenTok::RoleConstants::MODERATOR
NameError: uninitialized constant OpenTok::SessionPropertyConstants
```

Fixed this by autoloading the 2 modules that define these constants.