[MRG] Proxy support + pluggable cache

Proxy support is implemented in the following way in this PR:

1) proxy factories and caches are pluggable (per-request), so it is possible to use proxy and cache implementations other than bundled in splash;

2) there is a default SplashQNetworkProxyFactory that can load proxy rules (whitelist/blacklist) from ini files per-request.

See #18.