add Multidict

[wildart]

Multi-valued dictionary implementation based on Dict{K,Vector{V}}. It behaves as normal dictionary only

• setindex! appends new elements to the value vector
• pop! removes the last element from the value vector & if the value vector is empty removes key

Some additional functions were added

• enumerateall creates enumerator through all (k,v) pairs in the dictionary
• count counts all values in the dictionary ( length returns number of keys)
• size returns number of keys & values in the dictionary

[wildart]

I wonder maybe size would be better way present element count information, so it would return

(#key, #value) =  size(d)

[IainNZ]

It'd be good to somehow knock this and #88 at the same time

[StephenVavasis]

@IainNZ, @wildart, @kmsquire

I can add analogous functions to mimic this functionality to SortedMultiDict, but I have a few concerns:

m[k] = newitem: the proposal here is that this operation acts like a 'push!' instead of an assignment statement, which seems to me inconsistent with the use of assignment elsewhere in Julia. Perhaps '+=' instead of '='. What do other readers think?

MultiDict <: Associative: MultiDict does not implement the Associative interface, so I think this subtype declaration should be omitted

[malmaud]

Ping @wildart, would be useful to have this. I vote for using insert!(m, k, newitem) (as SortedMultiDict does) instead of m[k]=newitem to be more consistent with expectations of assignment.

[malmaud]

Superceded by #115