Remove Mail Packages

ee stack remove mail
````
This command will remove all mail server packages keeping configuration files as it.

Sample output:
```bash
^_^[[email protected]:~]# ee stack remove mail
remove Dovecot package, please wait...
Reading package lists...
Building dependency tree...
Reading state information...
The following packages will be REMOVED:
  dovecot-core dovecot-imapd dovecot-lmtpd dovecot-managesieved dovecot-mysql
  dovecot-pop3d dovecot-sieve
0 upgraded, 0 newly installed, 7 to remove and 59 not upgraded.
After this operation, 11.7 MB disk space will be freed.
[...]
.
.
[...]
Removing Roundcube dependencies, please wait...
Removing Roundcube, please wait...
Removing unwanted packages, please wait...
[...]
.
.
[...]
Removing libunix-syslog-perl (1.1-2build5) ...
Removing pax (1:20120606-2+deb7u1) ...
Removing re2c (0.13.5-1build2) ...
Removing spamc (3.4.0-1ubuntu1) ...
Removing librpm3 (4.11.1-3) ...
Removing librpmio3 (4.11.1-3) ...
Removing liblua5.2-0:amd64 (5.2.3-1) ...
Removing libnss3:amd64 (2:3.15.4-1ubuntu7) ...
Removing libnss3-nssdb (2:3.15.4-1ubuntu7) ...
Removing libnspr4:amd64 (2:4.10.2-1ubuntu1.1) ...
Processing triggers for man-db (2.6.7.1-1) ...
Processing triggers for libc-bin (2.19-0ubuntu6) ...
Successfully removed mail server packages