Its absolutely a hack, but that's what I have the energy for right
now.

Closes #108, #106

This will ensure that end-users who are requiring that file don't
have runtime errors.

Because we're required to build a C extension in order to do so
(don't ask, its a long story), use that C extension to make the
path provided at build time available at run time.

Currently looks at the value of `FREEDESKTOP_MIME_TYPES_PATH` and
in `/usr/share/mime/packages/freedesktop.org.xml`, which is the
path you would expect to find that file at on a typical Linux
system.

This is the first step in removing the dependency on distributing
freedesktop.og.xml as part of the gem.

This allows it to be called at runtime.

It turns out that the position of '[Content_Types].xml' is not fixed in
this format, and whereas MS Office adds it in the beginning in general,
when files are generated using Google Docs, or RubyXL, this is not
necessarily true. They might add it at the end, so all these would be
detected as application/zip.

This fixes #39

Previously if we passed in an IO object which doesn't respond to #seek,
MimeMagic would read the whole IO into memory. And this is exactly the
type of objects that the Shrine file upload library deals with.

However, we can also "seek" by blank-reading that many bytes, and always
rewinding to the beginning, which is what we've implemented in this
change.

This change also doesn't require IOs to respond to #binmode.

If you speed up detection of common MIME types, you speed up MIME type
detection in general.

Previously we had to separate branches which did the same thing, one was
for IOs and other was for strings. However, if we wrap the string into a
StringIO, we can use the same branch for both.

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

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