Make public key server configurable

[lazlolazlolazlo]

User should be able to choose which key server to use for public key search.

[niklasfemerstrand]

Fix sloppy HKP security through #65 before implementing this. Give the users the option of importing from remote PKS only when the user optionally chooses to query remote PKS'.

[jseidl]

I believe this should have a structure similar to this:
(pseudo php code)
$hkp_servers = array(
'local' => array('http://server1:port','https://server2:port'),
'remote' => array('http://server3:port','https://server4:port'),
);

And the PKS source (local / remote) on the dialog

Comments pls