Pylext

Fixed problem in PyPi source distribution, added classifiers.

Added Jupyter Notebook support for PyLExt.

1. Load Jupyter magic commands for PyLExt:
python
%load_ext pylext.magics

2. Running Jupyter Notebook cell in PyLExt context:
python
%%pylext [-d] [-e] [<context>]

where
- `-d` enables debugging mode
- `-e` enables only marcos expanding mode
- `<context>` is a PyLExt context name, which is `default` by default.

3. Clear PyLExt context in Jupyter Notebook:
python
%pylext clear <context1> ... <contextN>

**PyLExt** provides add in Python language:
- new operators with specified priorities,
- syntax extensions given by [LR(1)](https://en.wikipedia.org/wiki/Canonical_LR_parser) grammars,
- macros associated with new syntax rules,
- new token types and literals.