Make Symbol Comparable

I recently noticed that `Symbol` is not [`Comparable`](http://ruby-doc.org/core-2.1.0/Comparable.html) in Rubinius. This feature has existed in MRI since version 1.9.

###### MRI 1.8 and lower
```ruby
:baz.is_a?(::Comparable)  #=> false
:baz.between?(:bar, :foo) #=> NoMethodError: undefined method `between?' for :baz:Symbol
```

###### MRI 1.9 and higher
```ruby
:baz.is_a?(::Comparable)  #=> true
:baz.between?(:bar, :foo) #=> true
```

###### All versions of Rubinius
```ruby
:baz.is_a?(::Comparable)  #=> false
:baz.between?(:bar, :foo) #=> NoMethodError: undefined method `between?' on baz:Symbol.
```

`Symbol` already implements the spaceship operator (`<=>`), so the actual solution to this issue is a one-liner (`include Comparable` on line 2 of `kernel/common/symbol.rb`) but, I noticed a lack of specs for the various `Comparable` methods (`<`, `<=`, `==`, `>`, `>=`, and `between?`).

Before mixing-in `Comparable` to `Symbol`, I wanted to make sure it was properly tested in other core classes that mix-in the module, notably `String`, `Numeric`, and `Time`. Then, I plan to add failing specs for `Symbol` before making the one-line change to resolve this issue.

Since this is my first real contribution to Rubinius, I’ve submitted my earliest commits as a work-in-progress to start getting feedback. Please do not merge this pull request until I’ve indicated that it’s ready-to-go but feel free to look over the current progress and provide feedback.

Thanks!