[BioPython] More string methods for the Seq object

[Peter]

Dear Biopythoneers,

On bugzilla I've put forward a couple of suggested string-like methods
which I would like to add to the Seq object.

Bug 2351 comment 15 - adding a split method
http://bugzilla.open-bio.org/show_bug.cgi?id=2351#c15
Here I have suggested the separator be non-optional (for strings this
defaults to white space)

Bug 2596 - adding strip, rstrip and lstrip
http://bugzilla.open-bio.org/show_bug.cgi?id=2596
Here I have suggested these default to stripping gap characters (for
strings these default to stripping white space)

Any comments?  Positive or negative feedback is welcome.

Peter