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"]).
Merge pull request #1 from a4lg/feature/alternative-profile-image-sizes

Add feature of alternative profile image sizes