add version-strings into the module

The version string is in the module as __version__ available.
In the doc the version will be automatically parsed. The version comes
from the installed module if on read the docs or directly from
setuptools_scm if you are working locally.
The CLI will issue the version string if you call it with the option
'version'. The CLI does not accept commands like '--version', since this
would not be conform with the rest of the interface (and argparse).

The configuration for loading the version-string is adapted from
pimutils/vdirsyncer. It autogenerates a version string setuptools_scm at
install-time and then adds it to the __version__ member in __init__.py

I adapted the GitHub-template with a fitting comment and bumped the
changelog.

closes #141

src/escpos/__init__.py

@@ -1 +1,19 @@
+# -*- coding: utf-8 -*-
+"""
+python-escpos enables you to manipulate escpos-printers
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
 __all__ = ["constants", "escpos", "exceptions", "printer"]
+
+try:
+    from .version import version as __version__  # noqa
+except ImportError:  # pragma: no cover
+    raise ImportError(
+        'Failed to find (autogenerated) version.py. '
+        'This might be because you are installing from GitHub\'s tarballs, '
+        'use the PyPI ones.'
+    )