Allow Numeric#to_s to convert to different radices
Make Numeric#divmod function like Ruby's version

JavaScript modulo works differently from Ruby's version when either side
is negative. This would cause some pretty frustrating errors in Opal
apps. This commit fixes that with a special case if either side of the
expression is negative.
Add Numeric#to_s spec from RubySpec's Fixnum#to_s
Use radix conversion in JS
Remove DIGIT_MAP constant

No longer necessary since we're not converting number bases in Ruby.
Remove unnecessary explicit return
Merge pull request #362 from jgaskins/numeric-to_s-with-radices

Allow Numeric#to s to use radices