Added proxy support

I'm adding proxy support to the creation of the DynamoDB client. 

In the credential map two new keys :proxy-host and :proxy-port can be specified. If they exist, a ClientConfiguration instance is added to the constructor of the AmazonDynamoDB object. 

E.g.:

{:access-key "AAA" :secret-key "ZZZ" :proxy-host "proxy-us.intel.com" :proxy-port 911}