Add copy(img, data::AbstractImageDirect) (Fixes #139)
Fix #139
Merge pull request #140 from timholy/fix_double_Image

Add copy(img, data::AbstractImageDirect) (Fixes #139)
Added non-maximal suppression function (nonmaxsup)
Refactor nonmaxsup

* Renamed nonmaxsup -> thin_edges_nonmaxsup, added thin_edges_nonmaxsup_subpix
* Added Offset type which contains the sign, integer magnitude,
  and fractional parts of the offset.
* Split out functions for discretizing angles, interpolating offset
Nonmaximal suppression updates

* Rename Offset -> PointOffset
* Continued refactoring common code into functions
* Allow specification of discretization angle step size
* Transpose subpixel points if Image is column-major order
* Use copy(img, out) to properly return an Image
Nonmaximal suppression updates

* Re-merge the two versions of thin_edge_nonmaxsup() per
  Tim Holy's suggestion of a core function and two thin wrappers
* Renamed PointOffset -> CoordOffset (because that's what it is)
* Define `zero` for Base.Graphics.Point if necessary
Nonmaximal suppression updates

* Include "nonmaxsup" in Images.jl
* Export thin_edges_nonmaxsup{_subpix}
* Also fix subpix calculation.