Adding support for Proxy options

Adding parsing of proxy options: 

Proxy address : proxy_addr
Proxy port : proxy_port
Proxy username : proxy_user
Proxy password : proxy_password

For instance : 
```ruby
  config.logger = Logglier.new("https://logs.myserver.com/", :threaded => true, :proxy_addr => "X.X.X.X", :proxy_port => 3128)
```