Adding support for metadata in webp images.

Pillow now uses the webpmux library to envelop the webp images in RIFF. This allows for easy support of exif and icc_profile metadata.

Also included tests that verify compatibility with jpeg for exif and icc_profile metadata.

If the user does not have webp with webpmux enabled, pillow will fall back to the previous approach, meaning no exif or icc_profile metadata will be read or written to.
Merge pull request #271 from heynemann/master

Adding support for metadata in webP images.