bug 3204: implement request signing for Amazon Web Services

bug 3204: implement request signing for Amazon Web Services

After 2009-08-15, Amazon Web Services will expect that
all requests to the Product Advertising API, which is what
Koha uses for retrieving reviews and other enhanced content
from Amazon, include signatures.  This patch and
subsequenct patches implement this functionality.

What this means in practice (assuming the user has elected
to use any enhanced content from Amazon) is that

[1] The user must get a Amazon Secret Access Key.  This can
    be done by logging in to the user's AWS account
    at (e.g.) http://aws.amazon.com/, going to the 'Access Identifiers'
    page, and from there retrieving and/or creating a new Secret
    Access Key.

[2] The contents of the Secret Access Key should then be
    entered into the new AWSPrivateKey system preference.

Once that is done, grabbing reviews and table of contents from Amazon
should work as normal.  If the user doesn't do this before 2009-08-15,
reviews and TOCs will no longer be supplied from Amazon, although there
should be no crashes - the content will simply not show up.

Note that the requirement to sign requests does *NOT* appear to apply
to simply displaying book covers from Amazon.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>