Simplifies HTML parsing and serialization.

[diegoreymendez]

This PR:

• Tones down the use of the Converter protocol, since it's really not bringing the advantages I hoped it would. We could eventually end up removing it entirely but that's outside the scope of this particular PR.
• Renames some classes to make them more intuitive:
  • InHTMLConverter -> HTMLParser
  • OutHTMLConverter -> HTMLSerializer
  • NSAttributedStringToNodes -> AttributedStringParser
  • HTMLNodeToNSAttributedString -> AttributedStringSerializer
• Relocates several files.

How to test:

Review the new hierarchy by opening the project and looking at it.
Make sure there aren't duplicated files.
Run the unit tests.

Integration:

I've tested and none of the changes in this PR affect our current WPiOS integration.

[SergioEstevao]

Good simplification !