[WIP] Correct resolution of local resources/images

[thc202]

This is still work in progress (missing test cases and docs), sharing before being finished to get feedback about the implementation/logic when resolving the local links.

The checkers MissingImageFilesChecker and MissingLocalResourcesChecker are now using two directories to resolve the links, the base directory (sourceDir) to resolve absolute links (ones that start with /) and the directory where the file is to resolve relative links (file.html, ./file.html, ../file.html).

The directory where the file is is obtained from the HtmlPage, which is now exposing the File it belongs to.
Both checkers end up doing the same logic so it might worth extracting a base class.

Fix for #210 and, eventually, #153.