Modified the osx instructions

Among the other issues discovered when installing pond using the osx
instructions: the gtk+3 settings have to be applied before installing
gtk+3 or you will get errors.
cli: improve delete command

 - add support for deleting drafts, received, and sent messages
 - require confirmation for deleting messages as well as contacts
 - doesn't delete pending outbound messages - they must be aborted instead
threat model: two more things contacts can do
cli: correct table heading for help --all
cli: add retain and dont-retain commands

note that automatic message deletion currently only happens in the GUI, but I
figured that being able to set the retention flag in the cli is useful anyway.
Added system.IsSafe() for OSX; editstate now runs on OSX
Hopefully clarify the difference between GTK+ Quartz and XQuartz.
Merge branch 'savagejen-master'
Move (dont-)retain commands to non-subprocess switch.
Merge branch 'leif-master'
Avoid casting in type switch and don't crash when deleting drafts without a destination.
Merge branch 'leif-cli-delete-command'
Merge branch 'leif-threat-model'
Merge branch 'leif-cli-help'
cli: swap order of inbox and outbox in status

I want to see the inbox more often than the outbox, and a long outbox causes me
to have to scroll up to see the inbox if the inbox is displayed first.
Avoid SafeTempDir with odd permissions.

We actually need rwx permissions, so check for it.
Reverse order of cli global/context help output

Fixes #70
Don't panic on gtkspell errors

gtkspell.New() returns an error if it fails to find a dictionary for the
current locale. Instead of panicing, just print the error to stderr and
continue without the spell checker.

"Fixes" issue #68
Merge branch 'ebfe-tmpdir-perms'
No need to call error.Error() when formatting as a string.