Any label color

A while ago new colors for labels were introduced as discussed here: https://github.com/dillenmeister/Trello.NET/issues/57. Potentially this is also a root cause of https://github.com/dillenmeister/Trello.NET/issues/53. Moreover, now unlimited number of uncolored labels can be attached to a card. JSON response for this card looks like:

    "labels":[
     {
        "id":"54720d01bafdf2374fc135d1",
        "idBoard":"50d6b44e8fab449f7800531f",
        "name":"some uncolored label",
        "color":null,
        "uses":1
     }]

I replaced `Color` enumeration with a class, so now any new color doesn't break an application and cards with uncolored labels can be deserialized.