Specs and implementation of Enumerable#to_h, Array#to_h
Enumerable#sort works even if #to_a returns a frozen Array

Ruby's documentation on Enumerable#to_a does not suggest that the returned
Array must be mutable (only that it must contain the items in the Enumerable).
In fact, Array#to_a itself can return a frozen Array. Therefore, it is better
to account for Enumerables which return a frozen Array from #to_a.
Add spec ensuring Enumerable#sort works even if #to_a returns a frozen Array
Make rb_funcall3 disallow private methods
Add specs for rb_funcall3
Remove 'register' keyword from st.h in C API

'register' keyword is meaningless in modern compilers and it causes
deprecation warnings in C++11 compilers.
Format README in Markdown
Clean-up line breaks
Make LICENSE a link
Create links to other repo resources
Syntax highlight command line examples
Add Table of Contents
Clean-up; Add punctuation
Add link to Rails
Tidy up 'Keyword arguments' link
Format CONTRIBUTING in Markdown
Add build status indicator
Add CodeClimate badge
Prioritize sections in README
Format Changelog as Markdown