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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Regenerated tables from mime database v1.0

Regenerate tables with 1.8-compatible encoding