update Signal documentation

* remove _splat.Signal docstring as it's not handled well by
  Sphinx
* create new class splat.Signal in __init__.py which inherits
  from _splat.Signal and put the class docstring there
* add doc section in general functions & classes for splat.Signal
* improve introduction to signal concept and add vibrato example
review and improve documentation

* review wording and fix typos
* update arguments descriptions in docstrings
* add a few properties with docstrings
update version to v1.2