[BioPython] More string methods for the Seq object

[Peter]

> I assume you [Bruce] are agreeing with ... follow[ing] the
> string defaults of white space for stipping or splitting (for
> consistency, even though this won't typically be useful for
> sequences).  On balance this would probably be best from
> a principle of consistency and least surprise for the user -
> I'll update the patches.

New patch for Seq object split, strip, lstrip and rstrip methods on
Bug 2596 which follows the python string defaults (splitting on or
stripping of white space).

http://bugzilla.open-bio.org/show_bug.cgi?id=2596

If everyone is happy with this, I can extend the unit tests and
documentation too.

Peter