tests: Move FakeLinkResolver into a separate file
Fix StructuredText#asHtml's use of LinkResolver

The LinkResolver is now an object (with its own
class) thus it should not be used as a function.

Should fix #18
Fix Document#asHtml (with links in StructuredText)

LinkResolver was only provided to DocumentLink, so
links nested in a StructuredText (for instance)
didn't receive it.
Provided linkResolver can now be a function

The LinkResolver can (and is) now used as a function,
so if an actual function is provided instead of it,
the code will code as expected.

The LinkResolver class is still useful:

  - to be used as an helper
  - because closures are not fully supported
Merge pull request #19 from dohzya/feature/structuredtext-linkresolver

Feature/structuredtext linkresolver