Reorganize the code into separate files

While trying to figure out how the gem internals work I noticed that all the code was in a single file.

I reorganized everything into separate files and got rid of Jeweler now using the more up-to-date Bundler for releasing.

No code was changed, except for the definition of Gnuplot::Plot::QUOTED.

Used the current test suite to make sure everything is working as expected.

Later on it might be a good idea to separate the test files too.