Dev: Minor changes to reduce code size.

- This commit trims about 400 bytes off the min library size
Fix: Custom length menu was broken

- If you provided your own <select> list for the lenght language option,
  DataTables would still append its own and ignore your custom one

- See thread 20548
New: Add `preAjax` event so plug-in authors can add Ajax params
DataTables 1.10.0-rc.1!