ENH: Raise informatively if ImageSequence finds no files.
Merge pull request #90 from danielballan/io-error

ENH : Raise informatively if ImageSequence finds no files.
FIX: Spell grey consistently (following scikit-image).
FIX: Make as_grey work on frames shaped like (3, x, y).
FIX: Use TiffFile.series to return unreduced frames only.