Add /v2/mounts and /v2/mounts/skins

[darthmaim]

We haven't mentioned mounts here on github yet, so I thought I would go ahead and make a PR.

This adds /v2/mounts and /v2/mounts/skins.

// GET /v2/mounts
[ "Raptor", "Springer", "Skimmer", "Jackal", "Griffon" ]
 
// GET /v2/mounts/Raptor
{
    "id"           : "Raptor",
    "name"         : "Raptor", // localized
    "mastery"      : 14, 
    "default_skin" : 1,
    "skins" : [
        1,6,7,8,9,10,11,12
    ],
    "skills" : [
        41378, 40409
    ]
}

// GET /v2/mounts/skins
[ 1, 2, 3, ... ]
 
// GET /v2/mounts/skins?ids=1,6
[{
	"id"    : 1,
	"name"  : "Raptor",
    "icon"  : "https://render.guildwars2.com/...",
    "type"  : "Raptor",
    "flags" : [
        "ShowInWardrobe"
    ]
}, {
	"id"    : 6,
	"name"  : "Spooky Raptor Mount",
    "icon"  : "https://render.guildwars2.com/...",
    "type"  : "Raptor",
    "flags" : [
        "ShowInWardrobe"
    ] 
}]

Not sure if mixing string ids and subendpoints is the best idea, maybe /v2/mount/types and /v2/mount/skins are better?

Not included in this PR (yet?) are additional fields in /v2/account returning unlocked mounts and /v2/characters returning the equipped skins and dye channels (see also #610).

[apoch]

These endpoints have been implemented (as /v2/mounts/types and /v2/mounts/skins). We can either update the API doc in-house, or if you'd like, feel free to update this PR with the current endpoint description and I'll be happy to merge it in. Thanks!