Fix Linux installation, gem build, remove monkey-path

`summarize.so` was built in `./lib/summarize` under Linux machines -
the error doesn't occur in OSX. The fix is to create the
folder `./lib/summarize`, then `summarize.so` is correctly moved
to `./lib/summarize/summarize.so`.

As we needed to create the `./lib/summarize` folder, we decided to split
`./lib/summarize.rb` into several files for better organization.
Thus, we moved LANGUAGES and VERSION to the new folder.

This commit fixes the `cannot load such file -- summarize/summarize` error.
The error occurs when the gem is built with `gem build summarize.gemspec` if
`rake build` isn't called.
The fix is `require 'summarize/version'` instead of `require 'summarize'`.

After all that was done, we fixed the need of monkey-pathing std lib, now
the user can choose to opt-in the monkey-pathing by `require 'summarize/core_ext'`
otherwise he will use `Sumarize.summarize('text to be summarized')`
with the same params as the monkey-patched method.