Provide command-line interface

This makes it possible to call the script as follows:

python -m swagger_spec_validator "http://petstore.swagger.io/v2/swagger.json"

requirements-dev.txt

@@ -1,6 +1,4 @@
+.
 httpretty
 mock
 pytest
-jsonschema
-pyyaml
-six

swagger_spec_validator/__main__.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+
+from swagger_spec_validator.util import validate_spec_url
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.description = 'Check specification against the Swagger standard'
+    parser.add_argument('url', type=str)
+    args = parser.parse_args()
+    validate_spec_url(args.url)
+
+
+if __name__ == '__main__':
+    main()