Add support for 'http_proxy' and 'https_proxy' environment variables

These environment variables are somewhat standard and are respected by tools such as ‘wget’ on *NIX and Windows.

The wrapper script could detect these environment variables and set the appropriate Java system properties.

Linked to GRADLE-2393.

Raised another (slightly different, but related) idea:

As http_proxy env variable isn’t really a standard on Windows (e.g. ignored by browsers). The OS-level proxy configuration is standard there.