Add methods for working with usernames. (#897)

Added methods for working with usernames:

Check username (Validates a username and checks availability)
Update username (Changes username for the current user)

TLSharp.Core/TelegramClient.cs

@@ -234,6 +234,26 @@ namespace TLSharp.Core
             return (T)result;
         }
 
+        public async Task<TLUser> UpdateUsernameAsync(string username)
+        {
+            if (!IsUserAuthorized())
+                throw new InvalidOperationException("Authorize user first!");
+
+            var req = new TLRequestUpdateUsername { Username = username };
+
+            return await SendRequestAsync<TLUser>(req);
+        }
+
+        public async Task<bool> CheckUsernameAsync(string username)
+        {
+            if (!IsUserAuthorized())
+                throw new InvalidOperationException("Authorize user first!");
+
+            var req = new TLRequestCheckUsername { Username = username };
+
+            return await SendRequestAsync<bool>(req);
+        }
+
         public async Task<TLImportedContacts> ImportContactsAsync(IReadOnlyList<TLInputPhoneContact> contacts)
         {
             if (!IsUserAuthorized())