signUrl returns a URI

[jboynes]

Changes Storage#signUrl to return a URI rather than a URL as the value is typically given to another client to use rather than being resolved by the calling application.

[coveralls]

Coverage remained the same at 64.71% when pulling 11ce83c on jboynes:signing_generates_uri into 0cd6f98 on GoogleCloudPlatform:master.

[aozarov]

Both serialize and can transform to one another. Indeed Java URL implementation also provides functionality such as get the resource but the main reason I used it (vs URI) is that I feel it is much more popular/known (for various reasons including that URI came later) and corresponds to the GCS naming that is using URL.

When I search google "what is URL" or "what is URI" I get:

• URL ... "A URL is the fundamental network identification for any resource connected to the web"...
• URI ... "string of characters used to identify a name of a resource"..

So I think the main difference is that URL is more specific about being a "web" resource, which
is a better (more constraint) description in this case. Is the main argument against it is that in
Java they also chose to provide functionality to it? If so, I would have still kept it as URL (for naming
consistency and being more specific).

@jgeewax I don't have strong opinion in this case. Do you?
If we go with URI should we also update other part of the method (method name, java doc mention URL)...

[jgeewax]

If the distinction is as @jboynes said in his first comment, I agree that URI is the right choice. I also should note that AWS is using URIs also (take a look at AmazonS3URI)

[jgeewax]

I also agree that this PR should include changes to swap other uses of URL -> URI (if there are any).

[eamonnmcmanus]

From a practical standpoint, the java.net.URL class dates from the earliest versions of Java and is rubbish, so I would say that in general java.net.URI is preferred regardless of whether the thing in question is really a URI or a URL. But in that case I think the method name and documentation should also say URI. Otherwise it will be confusing.

[jboynes]

Removed use of URL from the doc and renamed the method.

[aozarov]

We decided to keep the URL. see #14 for more details.