Hatch linewidth for pdf and ps backends (#235).

I added a simple rc parameter (hatch.linewidth) that can be used to specify the width of lines in barchart/etc. hatch marks.  This was previously submitted to the sourceforge bug tracking system about a year ago.  

The hard-coded pdf default linewidth was 0.1pt, the hard-coded ps default was 0pt.  The patch sets the default across the two as 0.1pt.

Trying to get the issue number in here as #235