Unit tests for blaze.toplevel.open().  Don't pass yet.
Cleanup of blaze.toplevel.open().

Require a URI (rather than allowing uri=None implicitly).  Also fix
URI parsing to allow URI to specify on-disk relative and absolute
paths like:

  sqlite://foo.sqlite
  ctable:///tmp/mytable
  carray:///home/user/array
Give Table a datashape property to match Array and NDTable.