By storing bundles in a sorted dictionary (dictionary + list of keys),
the bundle does not need to be reparsed during installation if it has
already been initialised, and any additional options passed in the
original command are kept.

This changeset is also an initial attempt to stabilise the name vs
spec naming convention, where a bundle spec is whatever is used to
specify a bundle, be it a name, user/name, full uri, etc., and name is
the name of the bundle that will become a directory under the bundles
directory.

closes #1

This changeset allows the second parameter of the Bundle command (a
dictionary) to override bundle object fields that come from parsing the
bundle spec. This means that
 Bundle 'abolish.vim', {'name' : 'abo'}
will be installed in a bundle directory named 'abo', rather than
'abolish.vim', which is the name parsed from the bundle spec.