command.py: some fixes for handling of common-command initialization
command.py: job run: correct docstring: "heterogenous" => "homogenous"
command.py: fetch: use arg. to specify what to update, not where it goes
core.py, settings.py: make Package.fetch more flexible, robust

Default package repositories are now specified by both URL and branch,
to allow for using non-master branches by default.

Added `--ff-only' to the call for updating an existing package, to ensure
that nothing gets clobbered.
util.py: create_struct: allow creation of structure with ordered arguments

This should probably have been committed *prior* to the previous commit
(@c920f23)...
util.py: Table.append: use correct regexp group for field width
core.py: added support for documentation on options from `configure`
command.py: added `show options` to dump `configure` script options
util.py: Table: removed unused member variable
README.md: more updates, mostly re: build configuration