CHOP-63. Add a series of descriptive stats methods to KijiSlice.

The following methods now exist on the KijiSlice class:
* min/max
* mean
* standard deviation
* variance
There are two versions of each method:
* One that requires a function argument that returns a numeric value in case the
  underlying KijiSlice cell is a complex Avro type.
* One that assumes the underlying KijiSlice cell value is numeric (a convenience
  version of the above. Code won't compile if the underlying cell type is not numeric).

Reviewed by: kiyan, robert