Fix packaging and some code.

I read about this project today, and thought it's really cool. I'd like to play with academic papers for a toy project, and I'm sure many other PhD students would find this tool really useful.

I was very happy to see this is Ruby, and I was hoping I'd be able to use it inside a Ruby application.  The code doesn't seem in shape for that quite yet. I made a few changes to get it closer.

* I moved the files in `lib/` to `lib/pdf/extract`. Gems should generally not pollute the global `require` namespace.
* I updated the paths inside all the files to point to the new location.
* I added the gems needed to run `test/catalog.rb` as development dependencies in the gemspec.
* I created a simple `Gemfile` that references the gemspec, to make it easy to develop this gem and run its tests.
* I moved `assign.rb` and `train.rb` out of `bin/` and turned them into Rake tasks. It seems like they're used for development, so they shouldn't end up in the user's path when the gem is installed.
* I updated the code to work with the newer `pdf-reader` API and updated the version number. This should fix #4.
* I replaced [libsvm-ruby-swig](https://github.com/tomz/libsvm-ruby-swig) with [rb-libsvm](https://github.com/febeling/rb-libsvm). The former crashed on my setup (ruby 2.0.0 on OSX 10.9) and hasn't been updated in 2 years. The latter has been updated this year, has a newer libsvm and, most importantly, doesn't crash Ruby.

I was able to run `bin/pdf-extract` to extract titles and references from a PDF, and I was able to run `rake assign PDF=.....` to build training data from a PDF file. I think this means that my libsvm code changes are correct.