Add feature of alternative profile image sizes

User.ProfileImageUrl and User.ProfileImageUrlHttps returns "normal" variant of the user's uploaded image and there's no Twitter API to obtain other variants.
According to <https://dev.twitter.com/docs/user-profile-images-and-banners>, replacing URL string will simply work.
This commit adds two new extension methods GetProfileImageUrl and GetProfileImageUrlHttps which accepts alternative variant size as 'size' parameter (expected values are "normal", "bigger", "mini", "orig" and null-or-empty string [implies "orig"]).

CoreTweetSupplement に追加するとしたらこういう形の pull request で良いのかな?