add hist!, which accepts a pre-allocated array to store the output
add hist2d! that accepts a pre-allocated result array
add documentation for hist! and hist2d!
deprecate infs and nans

these functions are silly. we could also have fill, ones, and zeros
return BitArrys for Bool elements, and then we wouldn't need trues
and falses.
Merge pull request #6217 from lindahua/dh/hist2

Add functions hist! and hist2d!
introduce AbstractSparseArray. Close #6213.
clarify docs on unsafe load and store and pointer arithmetic
Merge pull request #6223 from lindahua/dh/abstractsparse

Introduce AbstractSparseArray. Close #6213.
Merge remote-tracking branch 'upstream/master' into appveyor2