* Fix swapped parameters in ArgumentException and ArgumentNullException cstrs

The constructors of ArgumentException and ArgumentNullException are inconsistent with each other, which results in their parameters being swapped here and there.

* Use named parameters for ArgumentException constructors

Cleans up some of the methods of EmbedBuilder to use simpler syntax as well

This commit rewrites the remarks section of GetMessagesAsync, as well as adding examples to several methods.

+ Geez, I didn't know there were so many.

* Change all Select(... as ...) to OfType

* Add changes according to 194a8aa427

* Add input validation for bot tokens based on their length

* Add token validation to BaseDiscordClient#LoginAsync

Adds a TokenUtils class which is used to validate that tokens are correct

* Revert changes to DiscordRestApiClient

* Add Unit tests to the TokenUtils class, fix a logic error that was caught by those tests

* Allow for API to throw exceptions

Moves the validation of tokens to be inside of LoginInternalAsync, and writes a Warning to the console when the supplied tokens are invalid

Added some miscellaneous missing xmldocs

Added some REST documentation

...because it isn't a class.

* Added GetJumpUrl() as an IMessage extension method

* Removed extra line for consistency

* Moved the namespace from Discord.Commands to Discord

* lint: remove eof newline

* lint: use TextChannel to get GuildID