Implements a basic function to check magic numbers 4 file types

File types are: tar, zip, bzip2 and gzip files. This is needed to make
the image fetcher a little bit smarter.
Integrates Unpackit which supports reading magic numbers.

Unpackit will read the file's magic number to better decide how to decompress
and unarchive such file.
Updates Godeps
Merge pull request #24 from cloudescape/magic-number

Integrates Unpackit, a smarter way of decompressing and unarchiving image files