avoid loose matching on msoffice formats in overlay.rb

Extend the range we search for MS files

Sometimes the first instance of the word/ or ppt/ is not in the first 2000 bytes - 5000 should cover it.

replace test images by images from public domain

The original images were licensed under CC-BY-SA-2.5, considered
non-free by the Debian project
https://commons.wikimedia.org/wiki/File:Phalaenopsis_%28aka%29.jpg
The new files
http://www.public-domain-image.com/free-images/flora-plants/flowers/orchid-flower-pictures/white-and-red-orchid
are licensed under the CC public domain

Handle .docx, .xlsx, and .pptx with extra magic

The freedesktop magic doesn't work for the Office 2007+ files, it
detects them all as application/zip. I have magic which detects them,
but it requires looking at 2,000 byte ranges, too much for their
project (which has to support things like file browsers). Since you
already warn that `by_magic` is slow, it'd be nice to support it here
at least.

This would allow the paperclip project to use mimemagic and stop
relying on the `file` binary:

  - https://github.com/thoughtbot/paperclip/issues/1530#issuecomment-43375497

Support is enabled through an overlay, required with

    require 'mimemagic/overlay'

Documentation is in the README

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Fixed for Ruby 2.0

Updated by_magic() to work with Ruby 2.0 - tries to call .b on string so they are compared as binary

Update README.markdown

Correct tabbing for markdown code segment

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