ArticleEnhancer: Transform relative to absolute URLs

By enhancing articles with the help of the ArticleEnhancer, relative links and images of the extracted sites become unaccessible. These two functions transform every relative "href" and "src" URLs within the extracted XPath-results into absolute URLs.

Example:
relative URL:
```
../a/relative/url.html?a=1#b
```
appropriate absolute URL:
```
https://www.explosm.net/all/312
```
resulting absolute URL:
```
https://www.explosm.net/a/relative/url.html?a=1#b
```