Removed handling of temporary directory

Added the --tmp flag for manual selection of the temporary directory, whose security should really be up to sysadmins when configuring operating systems. Default is ZSh TMPPREFIX usually set to /tmp/zsh which, if not existing, will be created as world writable with a sticky bit.

This commit also includes a cleanup of commandline options and a fix to swap check.
manpage updates
considerations on password bruteforce vulnerability on steghide
fix for #165
better changelog info
Avoid using a tempfile on every key decryption

A tempfile was often used by Tomb in order to parse the stderr output of
gpg and detect if the password is correct or not. The tempfile was not
holding any secret information (see #162) yet this is an improvement for
Tomb's deniability since there is now much less going on in the temp
firectory.
whitespace cleanup and correct indentation to 4 spaces (no tabs)
small security measure against exporting all vars
swish-e indexing change and whoami cleanup

swish-e now creates a .swishrc config file inside the tomb so that
users can tweak its configuration by hand.
whoami() is also cleaned up, pending a fix for slackware usage.