Simplify and modernize Base62

Hi JT,

I have made some modifications to simplify the gem and modernize it.

I made the following:

* Simplify gemspec, e.g. using git to list files
* Use Bundler for development dependencies, so you can just run `bundle` to install dependencies when developing. This also adds some gem tasks like `rake release` to git tag version and release to RubyGems in one go.
* Add a Rakefile, so you can run `rake test`, or just `rake` to run tests. This also enables testing with Travis CI.
* Simplify the tests to make them easier to read
* Separate and simplify encoding and decoding to ensure code (`PRIMITIVES`) are only written once
* And finally simplify and convert all text files to markdown, because this appears to be more widely used for gems today.

Hope you can use it. Please let me know if you need anything else.

Thanks,
/Lasse