Refs #67. Added a utility function to convert a dictionary into a sequence of html attributes
Refs #67. Sort the items so they are always in the same order
Refs #67. Cannot use ElementTree to generate elements because setting the text will escape any contained html
Refs #67. Updated test case
Refs #67. Stop using &nbsp; for tabs
Refs #67. Added pydocx-tab to the style definition
Refs #67. Documented pydocx-tab