Add class to generate bw file header

sbws/lib/v3bwfile.py

+# -*- coding: utf-8 -*-
+"""Classes and functions that create the bandwidth measurements document
+(v3bw) used by bandwidth authorities."""
+
+import logging
+from sbws import version
+from sbws.globals import time_now, SPEC_VERSION
+
+log = logging.getLogger(__name__)
+
+
+class V3BwHeader(object):
+    """
+    Create a bandwidth measurements (V3bw) header
+    following bandwidth measurements document spec version 1.1.0.
+
+    :param int timestamp: timestamp in Unix Epoch seconds when the document
+                          is created
+    :param str version: the spec version
+    :param str software: the name of the software that generates this
+    :param str software_version: the version of the software
+    """
+    def __init__(self, timestamp=None, version=SPEC_VERSION, software='sbws',
+                 software_version=version):
+        self.timestamp = timestamp or int(time_now())
+        self.version = version
+        self.software = software
+        self.software_version = software_version
+
+    def __str__(self):
+        """Return header string following spec version 1.1.0."""
+        frmt = '{timestamp}\nversion={version}\nsoftware={software}\n' \
+               'software_version={software_version}\n'
+        return frmt.format(**self.__dict__)