Move nikola/main.py to nikola/__main__.py

Which makes `python -m nikola build` legal and possible, with one little
addition.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge pull request #991 from getnikola/dundermain

Move nikola/main.py to nikola/__main__.py
fix integration texts for languages

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>