Added vector contracts and test cases from the Ruby source code.
Removed unused method.
Removed another unused method.
Added vector contracts and test cases from the Ruby source code.
Removed unused method.
Removed another unused method.
Removed unnecessary handling of numerics in op_subtract_postcondition for the VectorContract.
Merge branch 'vector' of github.com:isupeene/sparse-matrix into vector

Conflicts:
	src/contracts/vector_contract.rb
Changed the coerce_postcondition in the vector contract to specify that the two results must be multipliable.
Merge pull request #13 from isupeene/vector

Vector
Specified a generic const_arguments method in the contracts module which checks constness on the specified arguments only if they are cloneable, where cloneability is defined as the ability to clone oneself without throwing an exception, and giving a result which compares equal to the original.