Add a (unefficient) solution to problem 3

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Add a solution to problem 4

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Add a solution to problem 5

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Add a solution to problem 6

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Add a solution to problem 7

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Add clojure.contrib to dependency list in project.clj

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>