Speed up email header parsing

Use String#scan for parsing email headers to speed up alp startup.

Before:

[penberg@localhost alp]$ time ./bin/alp Cloudius

real	0m12.139s
user	0m11.386s
sys	0m0.327s

After:

[penberg@localhost alp]$ time ./bin/alp Cloudius

real	0m6.734s
user	0m5.717s
sys	0m0.302s

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Speed up email sorting

Use Message#unique_name for sorting to avoid expensive File#basename
call.

Before:

[penberg@localhost alp]$ time ./bin/alp Cloudius

real	0m6.294s
user	0m5.706s
sys	0m0.312s

After:

[penberg@localhost alp]$ time ./bin/alp Cloudius

real	0m3.995s
user	0m3.444s
sys	0m0.307s

Signed-off-by: Pekka Enberg <penberg@iki.fi>