Uncomment None constant

[kinseytamsin]

I'm assuming this was commented out to avoid shadowing std::option::Option::None. However, aside from the fact that this essentially excludes a symbol from the library bindings that should be included, this is really not necessary. There are other ways for client code to avoid this issue, such as avoiding importing x11::xlib::* or re-exporting either std::option::Option::None or x11::xlib::None under a different name.

[kinseytamsin]

I talked to a friend of mine about this pull request and realized that my first commit is potentially a dangerous change for existing code. Commit 35bcddc is an attempt to address this and make this PR mergeable without a major version bump (per SemVer requirements). Perhaps if this is merged then the feature-gating can be removed in 3.0.0.