Changed ImgType to ImgMode

Instead of specifying a single img type, this setting now allows
combination types to be specified - e.g. if different types should be
used if the image fits in memory or not, both types can now be
specified.
Converted size calculation to use a long

A long was being returned, but the counter variable was an int, which
could lead to overflow.
Added PlaneConverter interface

The PlaneConverter interface converts SCIFIO planes opened by Readers to
ImgLib data objects. This allows for specialized population for
different containers.

Provided default implementations for PlanarAccess and RandomAccess
containers.
Added new ImgOptions options

* ImgType -> ImgMode. Can now specify multi-modes. For example: the
  CELL_PLANAR ImgMode's equalsMode() method will return true for any
  other mode that shares either of the types (e.g. PLANAR or CELL).
* Added PlaneConverter options to provide ImgOpener with custom PlaneConverter
  implementations.