Any label color

[takemyoxygen]

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.