Make all imports relative.

This makes it possible to use imageio as a subpackage of
another library.

Note that there are some absolute imports in testing.py,
but these are only used by tests.
Fix conversion to uint8

Closes #44

New image_as_uint8() function that plugins can use to
convert image data to uint8.
trigger travis
Merge pull request #46 from almarklein/issue44

Fix conversion to uint8
Backport changes from v1.0+docs
Allow resources inside the package structure

We keep a special directory for local resources.
Then we can create wheels for distribution that have
the FreeImage lib and a couple of images inside this
directory.
add environment arg to disallow internet fecting. tests are aware of it
add images to wheel dist