ENH: Add SimpleITK IO plugin.

IO plugin for SimpleITK, http://simpleitk.org/

imread and imsave implemented. Tests based off the PIL tests.
Change type to ssize_t for all index and size variables
Change type to ssize_t for all index and size variables
Change type to ssize_t for all index and size variables
Change type to ssize_t for all index and size variables
Rename Py_ssize_t to ssize_t for consistency within skimage
Change type to ssize_t for all index and size variables
Change type to ssize_t for all index and size variables
Change type to ssize_t for all index and size variables
Change type to ssize_t for all index and size variables
Use np.intp rather than ssize_t for ndarrays
Replace ssize_t with Py_ssize_t globally
Fix index type errors in draw module
Remove print statement
Fix index type error in SLIC function
Fix index type error in GLCM
Improve performance and type usage for GLCM loop
Fix integer type bugs in feature detection algorithms
Improve code layout of probabilistic hough
Remove unused CAPI numpy import and extend cython directives