Remove Logger.assertTrue.

Use Guava's Preconditions instead, which throw appropriate exceptions
rather than AssertionErrors (whose idea was that?!)

Also fix weirdness in CommandArguments where start was allowed to be
1 greater than end when getting words.

Change-Id: Icce38e36da6507f645fb8b3b872dfe2c60c43ea7
Reviewed-on: http://gerrit.dmdirc.com/2961
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager