Start deprecation of python 2.6

TODO.txt

@@ -16,6 +16,7 @@ Version 0.14
 
 Version 0.13
 ------------
+* Require Python 2.7+, remove warning in `__init__.py`.
 * Remove deprecated `None` defaults for `skimage.exposure.rescale_intensity`
 * Remove deprecated `skimage.filters.canny` import in `filters/__init__.py`
   file (canny is now in `skimage.feature.canny`).

skimage/__init__.py

@@ -156,4 +156,9 @@ else:
         _raise_build_error(e)
     from .util.dtype import *
 
+
+if sys.version.startswith('2.6'):
+    warnings.warn("Python 2.6 is deprecated and will not be supported in scikit-image 0.13+")
+
+
 del warnings, functools, osp, imp, sys