Size - refactor sizing functions to reduce file size and modernise code

- The _fnCalculateColumnWidths is probably the oldest function in
  DataTables which has mostly reamined as it was 5 years ago! However,
  its time to trim it down a bit and optimise now. This rewrite has all
  the same actions, but results in a minified file which is about 700
  bytes smaller than before.
ize - refactor _fnInitialise for size. Only about 60 bytes saved
Dev: Fix type in last commit
Size - refactor Ajax server-side processing parameter builder