Proposal for Matlab style errorbars

[Ben Abbott]

I've modified __errplot__.m to produce Matlab style errorbars, and would like 
some feedback before pushing.

I'm not sure how the box style error bars (#) are supposed to look. For my 
setup they look identical to the normal ones (i.e. "#~>" looks like "~>"). 

Rather than duplicate the same "look", I made the box styles look like "boxes".

I haven't pushed this yet, but did add some demos to errorbar.m. I recommend 
pulling that change to make testing/demoing this changeset easier.

Of course, as an added bonus, this implementation works with the fltk backend.

Ben

p.s. I hope no one minds that I cc'd everyone involved in the discussion on the 
bug tracker.

changeset.patch
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