tests: Name all tests ".t.pl"

This way, everything (for example, common.inc) can have its proper
".pl" ending and syntax highlighting works properly in every editor.
tests: Add benchmark.pl - benchmark EncFS against eCryptfs

Example Results
* Seagate Barracuda 7200.9, model ST3250824AS
* Linux 3.16.3
* EncFS 1c5c75c44fd4ad83535280858c6be7a64b172f22

Test            | EncFS        | eCryptfs     | EncFS advantage
----------------|--------------|--------------|----------------
stream_write    |     32 MiB/s |     38 MiB/s | 0.84
extract         |  28744 ms    |  30027 ms    | 1.04
du              |    495 MB    |    784 MB    | 1.58
rsync           |   3319 ms    |  62486 ms    | 18.83
delete          |   6462 ms    |  74652 ms    | 11.55

(eCryptfs is very slow for stat() on a classical HDD)
Rename README to DESIGN.md and link to it in README.md

This document provides a nice overview and should be presented
prominently.
DESIGN.md: Add markdown headings
Put benchmark results into PERFORMANCE.md
benchmark.pl: Align numbers right in Markdown output
Mention PERFORMANCE.md in README.md
Merge pull request #38 from rfjakob/next

Add benchmark.pl