Improve Warnings on Probably-Wrong Image Inputs

[aleju]

Improve the errors and warnings on image augmentation calls.
augment_image() will now produce a more self-explanatory error
message when calling it as in augment_image(list of images).
Calls of single-image augmentation functions (e.g.
augment(image=...)) with inputs that look like multiple images
will now produce warnings. This is the case for (H, W, C)
inputs when C>=32 (as that indicates that (N, H, W) was
actually provided).
Calls of multi-image augmentation functions (e.g.
augment(images=...)) with inputs that look like single images
will now produce warnings. This is the case for (N, H, W)
inputs when W=1 or W=3 (as that indicates that (H, W, C)
was actually provided.)

• Add an assert in augment_image() to verify that inputs are
  arrays.
• Add warnings for probably-wrong image inputs in
  augment_image(), augment_images(), augment() (and its
  alias __call__()).
• Add module imgaug.augmenters.base.
• Add warning
  imgaug.augmenters.base.SuspiciousMultiImageShapeWarning.
• Add warning
  imgaug.augmenters.base.SuspiciousSingleImageShapeWarning.
• Add imgaug.testutils.assertWarns, similar to unittest's
  assertWarns, but available in python <3.2.

[codecov-io]

Codecov Report

Merging #594 into master will decrease coverage by 0.15%.
The diff coverage is 75.25%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
- Coverage   96.49%   96.35%   -0.15%     
==========================================
  Files          40       41       +1     
  Lines       14550    14648      +98     
==========================================
+ Hits        14040    14113      +73     
- Misses        510      535      +25