Add support for Git proxy through system config.

This adds support for overriding the HTTP proxy for Git by setting the
system configuration. It uses the git command itself to write to the
config file.
Add support for Subversion HTTP proxies.

I ran into this while attempting to build MRI 1.8.7 which seems to
checkout from the old Subversion repository. At any rate unfortunately
ti seems that Subversion itself doesn't have a `config` command so we're
left with simply writing it directly to /etc/subversion/servers.
Remove unnecessary requires
Use only `git config`, don't write into /etc/gitconfig directly
Disable git and svn configurations by default

Only do the magic if `config.git_proxy` or `config.svn_proxy` are
specified.
Add no_proxy/exceptions to svn_proxy config
Configure git and svn after each provisioner run
Add `host` and `port` to UserinfoURI
Use UserinfoURI in ConfigureSvnProxy

Fixes crashing on `nil` or `false`.
Fix line feeds in svn config

The configuration also allows empty values.
Add unit tests for git and svn caps
Fix git proxy config with falsy values
Merge pull request #49 from tmatilai/git_and_svn

Support git and svn configuration