Fix compilation.

After exposing string_piece in nonvirtual public API, linking broke.
The problem is that when compiling ner::tokenize_and_recognize,
string_piece from morphodita::string_piece was used. But library clients
know only about nametag::string_piece and therefore cannot link.
Big API change.

Remove tokenize_and_recognize. Instead, provide a tokenizer.

The string_piece, token_range and tokenizer are publicly exposed in
ufal::nametag namespace. They could be exported directly in the
ufal::morphodita namespace, but the authors believe the dependency
between the two libraries is needless.
Allow newTokenizer call to return NULL.
Add NumericTimeValue sentence_processor.
Refactor and update documentation.

- Create nametag_about containing basic information about MorphoDiTa.
  This is used in README, beginning of MorphoDiTa manual, on MorphoDiTa
  demo page and LINDAT/CLARIN pages.

- Create nametag_readme using nametag_about.

- Add support for underlining headers in txt formats. The underline must
  be present directly in .t2t file, it is removed in Makefile in the
  html targets.

- Ignore generated .txt and .html files.

- Rephrase the description of UFAL website. The real site that hosts
  the tool and the models will be LINDAT/CLARIN, nevertheless,
  ufal.mff.cuni.cz/nametag is the main website for the tool.
Offer the C++ library in the pre-build packages.
Move the model documentation to separate file.

This way, we can create documentation for the package containing only
the specific language model. To that end, nametag_model_about and
nametag_model_czech_readme are provided.