+ This commit reflects the new changes made to the Discord Application Developer Portal after its major update

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.

+ Original lines stated that sending a emoji wrapped in colon will not be parsed, but that was incorrect; replaced with reactions instead of sending messages as the example

* 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

Should fix issues in audit logs when deserializing overwrites and
similar types which we use a custom contract resolver for.

Nullable<T> is not valid for Attributes, this is my suggested fix.

* Initial commit

* Add alternative suggestion

* Fix typo

* Update README.md

Added some miscellaneous missing xmldocs

Added some class summaries

Added second sample for adding reactions

Added some REST documentation

* Don't attempt instantiation of an abstract module
* Attempt associating a TypeReader in case one is registered late (ie. OnModuleBuilding)