Added AT&T texting option, updated Instructions tab

-Added AT&T (Text) option to Cell Num field of Setup
--Allows messages to be sent as pure text instead of multimedia
--Compatibility with prepaid plans, text-only, etc
--Updated ExtraPhonePanel.java, Setup.java, Email.java
-Added explanation of differences between AT&T and AT&T (Text) options in Instructions tab
-Removed instructions for putting multiple numbers in one Cell Number line
--Still possible, just deprecated
-Changed version to 1.6.4