Add error when missing S3 credentials
Initial implementation of signed url

Needs testing and unit test
Allow signed url also for different HTTP verbs
Add test for signed url requests
Add type spec for signed_url exported function
Merge pull request #6 from tgrk/signed_urls

Signed urls
Explicitly test the content of response for signed urls.
Radically improved documentation.
Fix signed url construction for fakes3
Merge pull request #7 from tgrk/fix_signed_urls

Fix signed url construction for fakes3
Removed unnecessary eunit include from s3_lib.