[Fix #5] Add an extra example for String#format

The example showcases the use of named parameters.
Add Array#init
Add String#squish!
Release 0.0.5
Add Enumerable#take_last and Enumerable#drop_last
Add some new Enumerable extensions

* Enumerable#drop_last_while
* Enumerable#take_last_while
* Enumerable#several?
Add some new Enumerable extensions

* Enumerable#sum
* Enumerable#sproduct
Fix sproduct doc comment
Release 0.0.6
Optimize performance of drop_last_while
Code style