only generate lowercase usernames

As @KevCui mentioned in #26, having uppercase letters in an email is a little weird

tmpmail

@@ -89,7 +89,7 @@ generate_email_address() {
     # So charcters such as dashes, periods, underscore, and numbers are all deleted,
     # giving us a text which only contains lower case letters form A to Z. We then take
     # the first 10 characters, which will be the username of the email address
-    USERNAME=$(head /dev/urandom | tr -dc "[:alnum:]" | cut -c1-11)
+    USERNAME=$(head /dev/urandom | tr -dc "[:alnum:]" | cut -c1-11 | tr "[:upper:]" "[:lower:]")
 
     # Valid TLDS which 1secmail provides.
     TLDS="com net org"