Add support for sparse matrices

minimum(P, dim) does not work if P is a sparse matrix. With Julia 0.3, I get an
error

    ERROR: scalarmin not defined

As a temporary workaround, do not compute contraction factor for sparse matrices
Use sparse matrices

This results in a factor of 4 reduction in runtime
Saving the queueing model
Remove type annotations

Removing type annotations means that the same algorithms work for generic types
as well (e.g. Rationals and BigFloats).
Use .+ instead of +
Correcting a mistake in the code