A generic implementation of a k2-treap.
Added default constructor.
Cosmetic changes :)
Added tests for k2-treap

Fixed bug in dac_vector: Previously dac_vector only worked if the
bit-width of the maximum in the vector was smaller or equal to the
bit-width of the length of the vector.
Fixed case: n=1
Merge pull request #199 from simongog/k2_treap

Add K^2-Treap Implementation