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.
In the Go standard library ```http.DefaultTransport``` and ```http.DefaultClient``` are variables instead of constants.

It's convenient if you are changing the URL based on the environment and want all the clients pointing to the staging URL instead of configuring each one of them.