Do not rely in Link.Rel for values and parameters, allow configuring APIURL and workaround for random errors with schemas with references between resources

@cyberdelia I needed some modifications, so I just went ahead and implemented them.

This includes:
 - PR #26 (support for link.targetSchema)
 - [Allow configuring service's API URL](https://github.com/ernesto-jimenez/schematic/commit/c31814d49983bb0d314737028de3279fc3be0af7) in order to be able to use the same library with different environments by changing the URL.
 - [Workaround to prevent failure when there are references between resources](https://github.com/ernesto-jimenez/schematic/commit/048a1f9899598407ed5e645a7894b2da729b94c6).
 - Values and parameters not relying on Link.Rel: [full explanation, including caveats](https://github.com/ernesto-jimenez/schematic/commit/3b605c549ea3652ca64c1c4a34bcb508eee60f5c)

Let me know what you think.
The JSON Hyper-Schema API says ```instances``` indicates the following:

> indicates the target resource that represents a collection of instances of a schema.

I left this because otherwise by default ```instances``` links would not return collections.

 - ```Link.TargetSchema: nil```: would return the resource hosting the links, not a collection.
 - ```Link.TargetSchema``` with non-object types (e.g: a string or a number): would return the primitive rather than a collection too.

I thought it would be convenient, but I'm not actually hitting these cases, so I'm happy to remove this.