ipythonblocks: Replace xrange() with range()

`range()` is an iterator by default in Python 3.  In Python 2, the
iterator `xrange()` is slightly more efficient than the
list-generating `range()`, but I doubt anyone is making block grids
large enough for the difference to be significant.
Merge pull request #19 from wking/no-xrange

ipythonblocks: Replace xrange() with range()