Handle (authenticating) proxy setting

[espinielli]

It looks like git2r is not handling PROXY configuration as from libgit2 API:
(at least) two examples are described in #269 as symptoms of this deficiency.

For example git_remote_fetch calls in git2r code can have a git_proxy_options as part of its git_fetch_options argument in order to handle PROXY settings but that is not (yet) done and make certain use scenarios fail.

To summarize, it looks like there is the need to properly fill the git_proxy_options in git_fetch_options (passed in 3 function calls in libgit2: git_remote_fetch, git_remote_download and git_fetch_options_init) or directly (passed in 2 function calls in libgit2: git_remote_connect and git_proxy_options_init) to make git2r work behind an authenticating PROXY.