Fixes #25 and #41: whitespaces in filenames not tolerated.

URLs reaching BURL.{open,load,localize} must be
1) %-decoded and
2) gotten rid of any 'file:' prefix
before they can be given to any file I/O operation.
Merge pull request #71 from sjrd/issue/fix-25-and-41-filenames-with-spaces

Fixes #25 and #41: whitespaces in filenames not tolerated.
Numerics: Pow and Abs added; also Float trig. functions, etc.
Merge pull request #75 from sjrd/float-ops

Numerics: Pow and Abs added; also Float trig. functions, etc.