Proxy support via options hash

Hi! I needed Harvested to work from behind a proxy, so I added proxy support (as noninvasively as possible), specified in the options hash passed to `Harvest.client` and `Harvest.hardy_client`. For example:

```ruby
Harvest.client('yoursubdomain', 'yourusername', 'yourpassword', {
  :proxy => {
    :host => 'proxyhost',
    :port => 1234
  }
})
```

Also modified the tests to run through the proxy if `proxyhost` and `proxyport` are specified in `harvest_credentials.yml`.