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.'
+    )

src/escpos/cli.py

@@ -17,6 +17,7 @@ import argparse
 import sys
 import six
 from . import config
+from . import version
 
 
 # Must be defined before it's used in DEMO_FUNCTIONS
@@ -448,7 +449,7 @@ def main():
     # Allow config file location to be passed
     parser.add_argument(
         '-c', '--config',
-        help='Altnerate path to the configuration file',
+        help='Alternate path to the configuration file',
     )
 
     # Everything interesting runs off of a subparser so we can use the format
@@ -491,6 +492,10 @@ def main():
         action='store_true',
     )
 
+    parser_command_version = command_subparsers.add_parser('version',
+                                                           help='Print the version of python-escpos')
+    parser_command_version.set_defaults(version=True)
+
     # Get only arguments actually passed
     args_dict = vars(parser.parse_args())
     if not args_dict:
@@ -498,6 +503,12 @@ def main():
         sys.exit()
     command_arguments = dict([k, v] for k, v in six.iteritems(args_dict) if v is not None)
 
+    # If version should be printed, do this, then exit
+    print_version = command_arguments.pop('version', None)
+    if print_version:
+        print(version.version)
+        sys.exit()
+
     # If there was a config path passed, grab it
     config_path = command_arguments.pop('config', None)