Add configuration setting to set proxy server

[hannobraun]

See #147

[hannobraun]

Why did the bot close this?

[matteosuppo]

arduino-create-agent.zip

How to test without being behind a proxy:

• create a proxy.ini file with this content:

name = "with proxy"
httpProxy = http://your.proxy:port

• with only the default configuration open the debug console and launch downloadtool avrdude latest arduino replace
  it should succeed (because it's not using proxies)
• select the proxy configuration and launch the same command
  it should fail (because it's trying to use proxies)

Of course, if you are behind a proxy, it should be the other way around (it should succeed when it's using proxies)