Code organization

Hi Yoichiro,

I really like your gem, I'm using it to help me do some topic modeling with WordNet hypernym lookup. It does a really great job!! However, I wanted to contribute as I saw that the gem files structure was a bit different. I made some changes. I hope you like them, 

Cheers.

Changes:
- Reformatted the code - removed tabs and replaced them with spaces
- The Lemmatize class is inside a module
- The String class (core) extension is in a separate file core_ext.rb 
- Added license in the gemspec file
- Minor ruby style changes following this guide https://github.com/bbatsov/ruby-style-guide