Build reference using LDoc plus doc improvements

[mpeterv]

• Use LDoc instead of outdated LuaDoc;
• Reference options in luacov.defaults module;
• Document luacov.reporter module and the reporter extension mechanism;
• A lot of small doc fixes.

Docs have been rebuilt using LDoc from its master branch. Latest version from LuaRocks works almost just as well, it only doesn't render a reference to luacov.defaults.savestepsize from luacov.tick module description.

[mpeterv]

Merging this, any typos can be fixed later.

@hishamhm to update http://keplerproject.github.io/luacov/ with this I just need to merge this into gh-pages branch, right?

[hishamhm]

@hishamhm to update http://keplerproject.github.io/luacov/ with this I just need to merge this into gh-pages branch, right?

Yes!