Changed `sp-pair-overlay-face' to inherit from highlight face.

This provides a better "out of the box" experience. The previous face
was designed to 'be "pleasant" on the tango-dark color scheme'.

By using the highlight face it should be pleasant on any color scheme.

Signed-off-by: Arne Jørgensen <arne@arnested.dk>
Merge pull request #55 from arnested/sp-pair-overlay-face

Changed `sp-pair-overlay-face' to inherit from highlight face.