Add network delay / packet loss functionality to turtle

[mihails-strasuns-sociomantic]

@gavin-norman-sociomantic wrote:

Allowing a test suite to easily (i.e. by calling a single function, ideally) set up different degrees of network trouble, in order to test the application's robustness under these conditions.

Proposed approach was to use virtual network namespaces combined with iptables filters. However, this is not tested and very far from what turtle does currently, so keeps being delayed.