Meli refactor

[ghost]

Well, there are some changes here:

com.mercadolibre.sdk.Meli.java:

• Reorder methods: first the public methods, but getters. After, the gettes methods and at last, the private methods
• Use of the keyword final in methods arguments for immutable types
• Refactor the way constructors set attributes (the constructor with more args call the other that have less args)
• Creation of the private method executeHttpRequest(BoundRequestBuilder): four methods execute the exactly same piece of code: extract method
• Externalization of AuthUrls enum to a separated Java file -> com.mercadolibre.sdk.AuthUrls.java
• Creation and externalization of constants (a lot of) in a separated Java file -> com.mercadolibre.sdk.Constants.java. This one have two inner classes
• Creation of the private method getFromJson(JsonObject, String): extract method
• Removing the private method hasRefreshToken(): not used (since before the refactor)

com.mercadolibre.sdk.MeliException.java:

• Adding some constructors as Exception.

com.mercadolibre.sdk.AuthorizationFailure.java:

• Renamed to AuthorizationFailureException.java.
• Adding some constructor as Exception
• I thought this class could extends MeliException instead of Exception, but I was not sure about this.. So I leave it the way it was

README.md:

• The example of instantiation of Meli class was wrong. It doesn't compile this way. We need to force the first argument to Long: Meli m = new Meli(1234L, "a secret");

That is it! About the externalization of the enum... Maybe it could keep in Meli.java as a inner enum. I don't know... When I look to the way we use this enum, I don't understand why it need to be a inner enum and not a separated Java file.

Oh, I forgot: this is my first (real) contribution in a open source project. Sorry if I made some mistakes. Feedbacks are welcome.

[ghost]

Now I see I don't know why in README.md line 19, the dot was removed from github.com. Sorry.