Corrected dates in discussion

Date in discussion has been changed to display local date.
Added "5 minutes" and "1 week" expirations.
Added robots meta tags in html head in case ZeroBin is not installed at root.

From
https://github.com/elrido/ZeroBin/commit/2470871e709a227c72fe664143672b520ec8c62e
Allow discovery

Patch from
https://github.com/ic0nic/ZeroBin/commit/50d4fb9bf36ed154792e8a7d4d7d444f59581c74
Updated json checking.

- adapted to SJCL changed
- added entropy checking (from
https://github.com/vikstrous/ZeroBin/commit/f2ee2e8ba2f9c4cb1f16e81554c104109e202f24)
base64.js updated to 2.6

From https://github.com/dankogai/js-base64
Removed unused icon.
Added version to js/css assets URLs.

(in order to prevent some abusive caches to serve an obsolete version of
these files when ZeroBin is upgraded.)
base64.js downgraded from 2.6 to 1.7

because otherwise it would have broken compatibility with data files.
"Burn after reading" as a checkbox

"Burn after reading" option has been moved out of Expiration combo to a
separate checkbox.
Reason is: You can prevent a read-once paste to be available ad vitam
eternam on the net.
Removed dead code.
Clickable link color in syntax-colored pastes.
Small typo correction.
Added "Raw text" button.
Merge commit '6b089b4b856c5311d83bc6739024d32d1cbf34d6'
replaceState() changed to pushState()

so that the "Back" button works after clicking on "Raw text".
Merge remote branch 'sebsauvage/master'