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
Make NuGet.exe truly executable
Make XML file extensions in lower case
Remove Japanese comments and error messages
Add .gitignore pattern for MonoDevelop support
Downgrade dependencies for better compatibility
Add MonoDevelop solution and separate PCL
Remove trailing spaces
Merge pull request #3 from a4lg/fix/rewrite-portable

Rewrite for Portability