Expanded documentation on the S3 pre-signed url generator.

Also moved a few integer literals into constants for readability.
Added Aws::S3::Object#presigned_url method.

This method uses the `Aws::S3::Presigner` to generate pre-signed
object URLs.

    s3 = Aws::S3::Resource.new
    obj = s3.bucket('aws-sdk').object('key').

    obj.presigned_url(:get)
    #=> "https://bucket-name.s3.amazonaws.com/object-key?..."

URLs expire in 15 minutes by default. You can configure the
expiration period and provide additional params:

    # url expires in one hour, the file upload will be
    # publicly accessible
    obj.presigned_url(:put, expires_in: 3600, acl: 'public-read')

Fixes #134
Query serializer no longer sends `nil` as empty value.

Fixes #133