Added option for n-dimensional rectangular tiling

+ CL-option --polly-tile-sizes=<int,...,int>
  The i'th value is used as a tile size for dimension i, if
  there is no i'th value, the value of --polly-default-tile-size is
  used

+ CL-option --polly-default-tile-size=int
  Used if no tile size is given for a dimension i

+ 3 Simple testcases