Indicate when referenced schema isn't available

[chrahunt]

If a schema is being referenced via $ref and that reference cannot be resolved within the schema itself or from one of the schemas passed into the schemas property of the options, this needs to be indicated. A reasonable default would be to throw an error, but it would also be appropriate to indicate this issue at validation-time (as done in #46).

This would help take care of some of the issues referenced in #22.

[chrahunt]

A review of other JSON-Schema validators shows a pretty even split between the two possible ways of handling this:

1. throwing an error on compilation which is separate from the normal schema error-handling, or
2. reporting unresolved references in the same way as validation errors

Name	Error Time1	External Schema inclusion2
JJV	on validation	registration3
JSEN	on compilation	N/A
jsck	on compilation	both
Z-Schema	on validation	registration
tv4	on validation	registration
imjv	N/A	all-at-once4

It seems like the compilation-time error-throwing is present on the validation libraries that are closer in interface to this library, so it could be the least-surprising solution. Additionally, since the main schema and all possible remotes are given at the same time, we don't need to worry about the possibility of a referenced schema being supplied later.

¹ at what point unresolved reference errors are indicated
² interface used to add remote schema references
³ interface that allows 'registering' remote schemas using a method, e.g. in Z-Schema: validator.setRemoteReference(requiredUrl, JSON.parse(body));
⁴ interface like in imjv, where remote schemas are provided at the same time as the main schema to validate against