Add local address configuration in the HTTP client

Add a configuration option for specifying the local address to bind
sockets to in HTTP clients. The configuration is optional and will, as
before, default to not use a specific address when binding.

Binding to a configured address enables users of the library to, for
example, ensure that a specific network interface is used.
Merge branch 'bind' of https://github.com/protocol7/aws-sdk-java into protocol7-bind

Merging GitHub Pull Request #169
https://github.com/aws/aws-sdk-java/pull/169