doc: Vendorize alabaster Sphinx theme

Alabaster is now the default sphinx theme and is a significant
improvement over the previous default that it's worthproviding it when
unavailable (e.g. Sphinx <1.3).

.gitmodules

+[submodule "doc/alabaster"]
+	path = doc/alabaster
+	url = git://github.com/bitprophet/alabaster.git

alabaster @ aabda118

+Subproject commit aabda1182083d21b6d9bafb53ea8a99eb4be4cf0

doc/conf.py

@@ -29,6 +29,7 @@ pygments_style = 'tango'
 
 # -- Options for HTML output ----------------------------------------------
 
+html_theme_path = ['./alabaster']
 html_theme = 'alabaster'
 htmlhelp_basename = 'Haddockdoc'