Add sphinx bootstrap theme as git submodule

Thanks to kosiakk for the suggestion (via pull request github nwf/dyna#72).
This commit does not us to this theme, merely makes it available.  I'll
leave the actual flip to someone with a sense of aesthetics. :)
Tweak build process handling of git submodules
Failure on parse errors, issue #71

Solved by also searching for non-interval ranges

Conflicts:
	docs/sphinx/_themes/bootstrap
Tweak provided patch as per @timvieira

See https://github.com/nwf/dyna/pull/72#commitcomment-5504947