Refactor: Make information better accessible and code better readable

[busykoala]

Since most of the function are there to handle links and their validity we should introduce a link object having the attributes:

• Internal/External
• Origin
• Destination
• Status Code
• Content Type
• Response Time
• Header Location
• Error Message
• Is broken value

So that we could address the object nicely the obj class should also have methods to:

• auto complete all link information as soon as (and only once) we call it with corresponding object
• ask the link object if it is broken

This refactoring is especially important at places like: https://github.com/4teamwork/ftw.linkchecker/blob/63347068e01f86a79f138bf550b7f20890e46202/ftw/linkchecker/command/checking_links.py#L54-L58

Also we should loop through both in one loop instead of going through everything twice.

[busykoala]

Solved in #42