Fix conflict with Base.collect
Fix conflict with Base.norm
Merge pull request #11 from cdsousa/fixcollect

Fix conflict with Base.collect and Base.norm
Fix Array-Number deprecation in SymMatrix getindex.

Fixes

    julia> a = convert(SymMatrix, [sym"x"])
    julia> a[1,1]
    WARNING: A::Array - x::Number is deprecated, use A .- x instead.