Add #atom? implementation

Chnages were required to tbe adater and matchers to reflect that we return
Atom::TRUE rather than ruby true from query methods
Add Atom#eq?
Atom#eq? returns nothing when self or other is a number

This matches the definition of eq? in the box in the book, on the assumption
that numbers are defined as whole numbers.
Add specs for chapter two
Implement cond

We had to hack the adapter to get this working, and some questions were raised
about it's value now that our implementation is diverging from that which it
expects.

Note: else always evaluates to true in this implementation (which is at
least partially implied by page 23).
Merge pull request #4 from mocoso/fourth-meeting

Make the remaining chapter one examples, and some of the chapter two examples, pass

This work was done during the fourth meeting of the Little Schemer book club.