Preliminary Ruby gem for Flann

That went faster than I expected, but there are caveats. I haven't tested it quite as thoroughly as I'd like. I am not enamored with the way I'm passing the parameters struct. I may need comments from some other Rubyists as they try using this package.

Here are a few issues that need your consideration when you merge:
* The gem does not need to be built since it uses Ruby FFI. However, you may wish to include its specs in your makefile test target. The specs may be run as follows:
```
cd src/ruby
bundle exec rake spec
```
* The gem requires [NMatrix](http://github.com/sciruby/nmatrix), which means people need to be using the reference implementation of Ruby. It would not be too difficult to make it work with JRuby's MDArray (ask @rbotafogo) or with ruby-core's matrix.rb (which is slow).
* I have not yet tested gem building, but it should work. To do it, a user would need to do:
```
cd src/ruby
bundle exec rake gem
cd pkg/
gem install flann-1.8.4.1.gem
```
* A spec is set up to fail if the version gets incremented without changes being made to the gem. Be aware of that if you incorporate my rspec task into your make test target.
* I put in my own copyright information and used BSD 2-clause just like you. Feel free to add your own names to the notices in the Ruby-related files. I hope you don't mind, but we're trying to publicize our lab and facilities here, so it's important that our code be branded. =)

And thanks for encouraging this. I'm thrilled to be able to use your library, and will make sure to give credit. Flann might end up in space, if it isn't there already. =)

Addresses #165