Enhance s3 request timeout support

ReadTimeout is now a per Read timeout instead of an overal request timeout
which is now provided by RequestTimeout.

Added additional timeout options:
* WriteTimeout which is a per Write request timeout.
* RequestTimeout which is the overall request attempt timeout.
* ConnectTimeout which is the request connection attempt timeout.

Document each timeout option.

Also change New to used named parameters to avoid issues when adding new
S3 struct options.

This changes
Enhance s3 attempt strategy retry logic

Windows uses different error Op values for net.OpError errors so
request retries where not happening for read, write and connection
attempt errors.

Errors returned when operating on a Request.Body are wrapped in
url.Error. Unwrap these errors so retry logic works the same for
errors that occur when processing Request.Body.
Expose the attempt strategy used for s3 requests

The attempt strategy used by s3 requests can now be customised by
via S3.AttemptStrategy.

Export the default attempt strategy by New S3 objects as
DefaultAttemptStragety for user visability.
Ensure RequestTimeout cooperate with Read and Write timeouts

As Dailer SetDeadline is not seperate from net.Conn deadlines
ensure the two cooperate.

Also fix http connection reuse by storing client on the S3 object.