Beginnings of a proper API

First step towards defining a clean public API that exposes only the
necessary functionality. Currently we have the following three
functions:

DFGetFile - Converts a .docx file to a .html file.

DFPutFile - Updates an existing .docx file from a modified .html file

DFCreateFile - Creates a new .docx file from a .html file

Eventually these will be expanded to support other file formats as they
are incorporated into the library. The actual conversion functionality
is in the generateHTML and updateFrom functions, taken directly from
dfutil. Now, dfutil calls these public API functions instead of access
internal classes.

These function represent the simplest use cases of the library only;
namely conversion between different formats. They do not yet provide for
access to the structure of the document in its intermediate form.

We adopt the convention that the public APIs can be used from a consumer
by adding the following include:

    #include <DocFormats/DocFormats.h>