new branch with possible paragraph measurement fixes

The existing paragraph code has problems doing its width measurements. Consider

Paragraph("
<span fontName=\'Courier\'>Hello</span> World.<ondraw name="_onDrawFunc" 
label="end"/>",style)

where style sets up Helvetica 10 points as the base font; this ought to measure

stringWidth('Hello','Courier',10)+stringWidth(' World.','Helvetica',10)

ie 61.67; however, in our paragraph implementation it is thought to measure 
64.89 because we have a poor idea of which space to use so we measure a Courier 
10 space instead of Helvetica 10.

I have made a branch called para-measure-fix which attempts to fix this issue 
and I would ask interested parties to try this branch out. It includes an extra 
test in tests/test_platypus_breaking.py which attempts to check this.

You can see the results here

https://www.reportlab.com/static/test_platypus_paragraph_line_lengths-current.pdf

https://www.reportlab.com/static/test_platypus_paragraph_line_lengths-fixed.pdf

the modified test is here

https://www.reportlab.com/static/test_platypus_breaking.py

The branch code also allows some space shrinkage for tight paragraphs using a 
new rl_settings parameter spaceShrinkage (a value of 0.05 will allow spaces to 
be shrunk by 5%).
-- 
Robin Becker