Update NewClient handle credentials in the url

[opalmer]

This PR updates NewClient to handle credentials in the url:

client, err := gerrit.NewClient("http://user:secret@host:port/")

If the credentials are invalid then err will be set appropriately. This new function will be used in the docker_tests branch.

Another case were this could be useful is when a consumer of go-gerrit is using configuration files to drive their application. In these cases code could go from:

username, password := GetCredentials()
client, err := gerrit.NewClient(GetURL())
client.Authentication.SetDigestAuth(username, password)
// test auth

To:

client, err := gerrit.NewClient(GetURL())
// make sure err == nil

[opalmer]

@andygrunwald I know you're probably still out so I'll try to get to the point:

• This PR adds a function (NewClientFromURL) which allows credentials to be provided as part of the url. Other projects I've worked with have similar functions and I've already implemented this same function on another project using go-gerrit (so it would be nice to open source it).
• There are several tests which cover the new code including url handling, failed authentication and digest/basic/cookie auth too (which previously had no tests).

Unless you have some major objections, I'd like to go ahead an merge this so we can start to use it in the docker tests.

[opalmer]

Minor update, added credential handling directly to NewClient without breaking compatibility.

[coveralls]

Coverage increased (+7.0%) to 17.481% when pulling 4f29c44 on from_url into 2b0558c on master.

[andygrunwald]

I like the idea, because it breaks nothing :)
I will merge it. Thanks @opalmer