Update Carbonite to work w/ Clojure 1.2

[sritchie]

This required:

• Changing (def ^:dynamic some-var ...) calls to (def ^{:dynamic true} some-var ...)
• Putting docstrings on def metadata vs as the second form
• Checking for the existence of BigInt, and only including it where defined

I also added support for the lein-multi plugin. To test carbonite against all versions of clojure (1.2.1, 1.3.0 and 1.4.0-alpha3), run

lein multi test

To test against a specific version, run

lein multi test --with 1.2