Add optional proxy configuration to client.properties

(aka "Let the Monkeys rampage by proxy")

This patch adds the following optional proxy configuration to client.properties:

```java
simianarmy.client.aws.proxyHost=internal-proxy.example.com
simianarmy.client.aws.proxyPort=1234
simianarmy.client.aws.proxyUser=optionalUser
simianarmy.client.aws.proxyPassword=optionalPassword
```

Which are then used when configuring the various Amazon clients used within the project.

Fixes #150