Windows x64 support

This is an extension of the work done by @larskanis in #90 and make it compatible with the work done in tjschuck/rake-compiler-dev-box#2

While it compiles using `package_win32_fat_binaries.sh`, it still misses the `Gemfile` to be 100% automated.

This needs a bit of review and the following changes:

- [ ] Normalize on `Gemfile` and `gemspec` usage so task can be fully automated
- [ ] Upgrade to latest SQLite3 version (3.7.17 at this time)

We can't use the Knapsack package provided by RubyInstaller since the `x86-windows` version uses mingw-w64 specifics :cry: 

I'm opening this for final review of the changes and for approval.

/cc @tenderlove @larskanis

Thank you.