Bump version number to 0.2.15
Refactor spreadsheet grid access to a separate module
Updated mkminim to generate Minim direct from XLS filer (uses xlrd)
Bump mkminim version number (direct Excel support)
Add context protocol to HTTP_Session class
Use the content type string "*/*" for the default content negotiation case when no accept header is specified
Create commandf line utility to work with Overlay RO service.
Rename and update top-level README -> README.md
Code cleanup; refactor access to rosrs_uri and key
Rework setting of ROSRS URI so that arbitrary RO URIs can be accessed.

This change allows 'ro list' and similar commands to be used with ROs created by the "Overlay RO" service.
Resolve roverlay URI references against current directory. Update roverlay document to match implementation.
Tidy up error reporting for unavailable RO manifest
Update setup.py (and other files) to install roverlay command.

Note: the command line tools install dep[endencies needed to run them.
But the web frameworks used by the web servbices are not included, and must be
installed separately if the web services are to be used
(e.g. 'pip install pyramid' and/or 'pip install django')
Fix setup.py and test code so that ro-manager-test runs when installed to Python virtualenv
Fix setup.py to include all roverlay components.  Add installation documentation.

Updated setup.py to allow running roverlay server from installed software source.
The roverlay documentation has been updated with installation instructions and some
details about using Python virtual environments.
Update version history in documentation
Merge branch 'develop' - V0.2.15