Prevent inconstitent /data/trafic_limiter.php due to file read while writing
Potentiel security bug corrected

Bug reproduction: 1) paste texte containing html/javascript. 2) send 3)
clic "Raw text"  4) refresh: The html/javascript is interpreted instead
of just displayed.
Under some versions of Chrome, it happens without refreshing.
This bug was corrected.
Merge pull request #61 from jelhan/master

File lock for write on /data/traffic_limiter.php
Fix security issue
Stronger server salt

ZeroBin now generates a much stronger salt. This fixes issue #68
(mentioned in section 2.1 of https://defuse.ca/audits/zerobin.htm)
File locking on write by default.
Time attack protection on hmac comparison

This fixes issue 2.7 of https://defuse.ca/audits/zerobin.htm, and thus
(with commit a24212afda90ca3e4b4ff5ce30d2012709b58a28) also issue 2.8.
Merge pull request #75 from cese/master

Fix security issue (arbitrary file unlink)