Fixed: off-by-one in row calculation caused drawing problems with variable-height rows in CPTableView.

Previously, if the last row's height was increased, empty rows below would not be pushed down. This was due to an off-by-one in -_unboundedRowsInRect (now _exposedRowsInRect) which caused the incorrect rect of the last row to be retrieved.

This commit fixes that bug and also makes the following changes:

- Added FULL_ROW_HEIGHT(), ROW_BOTTOM() and HAS_VARIABLE_ROW_HEIGHTS() macros to make the code clearer.
- Documentation cleanup.
- Eliminated some intermediate variables.
- Rewrote -_rectOfRow to reduce redundant code and fixed some logic errors.
- Renamed some variables to make their intention clearer (to me at least).
- Added an out of bounds check in -rowAtPoint: before doing the relatively expensive binary search.
- In -noteHeightOfRowsWithIndexesChanged:, the first valid index is used instead of the first index (which might be invalid).
- Fixed a bug in -noteHeightOfRowsWithIndexesChanged: where row height was not set correctly if a row was not in the index set.
- Fixed incorrect use of CEIL in -_unboundedRowsInRect, it was wrapped around the divisor instead of the entire expression.
- Fixed drawBackgroundInClipRect: drawing one too many rows.
- Fixed -rowAtPoint not checking for x in bounds.

BREAKING CHANGE:
Previously, CPTableView -rowAtPoint would incorrectly return a valid row index when the x value was outside of the table's bounds. Now an out of bounds x value will return -1, which is consistent with Cocoa (and the intent of the method).