Wrap an AbstractDict to add getproperty support for Symbol and String keys.
julia> using PropertyDicts
julia> d = PropertyDict(Dict("foo"=>1, :bar=>2))
PropertyDict{Symbol, Int64, Dict{Symbol, Int64}} with 2 entries:
:bar => 2
:foo => 1
julia> d.foo, d.bar, d."foo"
(1, 2, 1)
julia> propertynames(d)
KeySet for a Dict{Symbol, Int64} with 2 entries. Keys:
:bar
:foo
julia> d.baz = 3
3
julia> d
PropertyDict{Symbol, Int64, Dict{Symbol, Int64}} with 3 entries:
:baz => 3
:bar => 2
:foo => 1