Error users earlier, add Single type

[GiggleLiu]

Thanks for @jon-wurtz 's feedback, I did the following changes in this PR.
Now if the input is an integer, it always return a TruncatedPoly type.

julia> ConfigsMax()
ConfigsMax{Single, true, false}()

julia> ConfigsMax(1)
ConfigsMax{1, true, false}()

julia> gp = IndependentSet(smallgraph(:petersen));

julia> solve(gp, ConfigsMax(1))
0-dimensional Array{TruncatedPoly{1, ConfigEnumerator{10, 1, 1}, Float64}, 0}:
{0010111000, 1010000011, 0100100110, 0101010001, 1001001100}*x^4

It also error users at input if the property if not computable

julia> gp = IndependentSet(smallgraph(:petersen); weights=rand(10));

julia> solve(gp, ConfigsMax(2))
ERROR: ArgumentError: Graph property `ConfigsMax{2, true, false}` is not computable due to having non-integer weights. Maybe you wanted `SingleConfigMax(2)`?

julia> solve(gp, ConfigsMax(0))
ERROR: AssertionError: k == Single || k > 0

Also updated some documentation.