use _version.py if not a .git folder

If someone install from a zipped source file,

    $ python3 setup.py sdist

and use

    $ pip-3.3 install source_file.zip[.tar.gz]

It cannot detect the git revision so will use an ambiguous version
number like 0.0.0.dev. However, after installation, if they get the
version through

    >>> import nextbiopy as nb
    >>> print(nb.__version__)

It will find the nextbiopy/_version.py and get the real version number,
which two ways get mismatched result.

Now it trys to retrieve the _version.py and sync the result.
fix instructions in _version.py

now using ``python3 setup.py --version`` to check version
simplify including version in __init__.py

A _version.py will always be included, drop the try ... catch statement.
Also always use absolute import
add version test