Added remaining markdown files to tocdoc.
Added brief descriptions for each sub-library.
Documented util.string.
Noted todos in util.string tests.
Made implode docs more precise.
Remove redundant using.md
Merge pull request #98 from joshuacc/tocdoc

Updated docs: sub-library descriptions & API for util.string
Made basic math functions variadic.
Made basic equality operators variadic.
Added missing tests for _.not
Made relative comparators variadic.
Made bitwise operators variadic.
Reorganize util.operators for consistency.
Merge pull request #99 from joshuacc/variadic-operators

Made (most) operator functions variadic
Wrote API docs for function.predicates.
Merge branch 'master' of https://github.com/documentcloud/underscore-contrib into predicate-docs
Merge pull request #101 from joshuacc/predicate-docs

API docs for function.predicate
Change isPlainObject to check for instances of Object.
Update docs for isPlainObject.
Wrote API docs for util.operators.