Pyconll

Fixed
- When used on Windows, the default encoding of Windows-1252 was used when loading CoNLL-U files, however, CoNLL-U is UTF-8. This is now fixed.

Added
- *Getting Started* page on the documentation to make easier for newcomers

Fixed
- Versioning on docs page which had not been properly updated
- Some documentation errors
- ``requests`` version used in ``requirements.txt`` was insecure and updated to newer version

Fixed
- The ``pyconll.tree`` module was not properly included before in ``setup.py``

Added
- ``pylint`` to build process
- ``Conllable`` abstract base class to mark CoNLL serializable components
- Tree data type construction of a sentence

Changed
- Linting patches suggested by ``pylint``.
- Removed ``_end_line_number`` from ``Sentence`` constructor. This is an internal patch, as this parameter was not meant to be used by callers.
- New, improved, and clearer documentation
- Update of ``requests`` dependency due to security flaw

Changed
- Removed test packages from final shipped package.

Added
- There is now a FormatError to help make debugging easier if the internal data of a Token is put into an invalid state. This error will be seen on running `Tokenconll`.
- Certain token fields with empty values, were not output when calling `Tokenconll` and were instead ignored. This situation now causes a FormatError.
- Stricter parsing and validation of general CoNLL guidelines.

Fixed
- `DEPS` parsing was broken before and assumed that there was less information than is actually possible in the UD format. This means that now `deps` is a tuple with cardinality 4.