Merge from HexKit 1.2

HexCoord:
- Added PolarNeighbor(), gets next/previous hex in same ring from
origin, faster than static AtPolar()
- Added AngleToHalfSextant(), HalfSextantToAngle()
- Improved behavior of CartesianBoundingRectangle(), bounds were
unnecessarily large in some cases.
- Unity 4.5.0 can serialize structs! HexCoord is now serializable.
- Readonly fields aren't exposed to the Inspector, so HexCoord is no
longer immutable.
- Updated Scale() to modify in-place. (Still returns, to avoid code
breakage.)

HexCoordinate:
- Deprecated for Unity 4.5.0+.