Use command 'identify' to retrieve the size of a local image

* Command 'identify' is provived by ImageMagick
Use function 'executable' to check if command 'identify' is available
Merge pull request #104 from dpino/fast-image-size

Use command 'identify' (if supported) to get the size of a local image
add zencoding_use_identify to disable. default is 1.
Merge pull request #1 from mattn/master

update