Added fixed corners for overlapping headers

Hi Allan,

I've added support for fixing the corners when adjacent headers are enabled (eg. 2 right columns fixed along with top row). Currently, the header with the higher z-element will overlap the corner and scroll into it when the table is scrolled; my changes create a new fixed corner which will behave properly when scrolling in either direction.

I did notice that the change that introduced the ability to fix multiple left or right columns (bb5fd5d8582e530521f0671034f8ca8f6b05b74e) introduced some inconsistency when the border-collapse:collapse option is set - in my tests on Chrome/Firefox the right fixed column ends up with a double-border. I am unsure how to fix this at this moment, and even unsure about the status of border-collapse compatibility with DataTables; based on some older discussions I've seen on your forum I see that issues with border-collapse have been around, but I'm uncertain if they're fixed or not at this point. Let me know if at this point everything else plays nicely with border-collapse.

- Thanks, Dmitry.