Docfix: most shape functions take Float, not number args

Looked at and manually tested all functions in Graphics.Collage with
arguments involving the type number.  Only the shape creation functions
(except for polygon) took Floats and not numbers as documented.
See
https://groups.google.com/forum/?fromgroups=#!topic/elm-discuss/sqFIM6A2zIU
for the discussion.
Merge pull request #52 from cstork/docfix

Docfix: most shape functions take Float, not number args