BUG: add signed flag to chars

Sign of char is undefined and there are numerous places where its sign
is important. Fixes tests with -funsigned-char.
Closes gh-1110
DOC: introduce variable num_peaks for reusability
Merge pull request #1136 from juliantaylor/signed-char

BUG: add signed flag to chars
Merge pull request #1137 from sciunto/doc

DOC: introduce variable num_peaks for reusability
Clear the blit background when the image changes
Merge pull request #1144 from blink1073/master

Clear the blit background when the image changes
Update find_contours docstring to use r/c notation

This update fixes #1140.

Other docstrings will probably need to be updated as well.
Merge pull request #1145 from jni/find-contours-docstring

Update find_contours docstring to use r/c notation
Use functools.wraps in default_fallback dectorator
Merge pull request #1151 from blink1073/default_fallback_doc

Use functools.wraps in default_fallback decorator

Fixes #1149, allowing functions wrapped by `default_fallback` to keep their docstring.