Fix bug exploited by FireShepard

[soldiermoth]

When using FireShepard it inserts a special string into a mock Facebook cookie which causes a syntax error on JSON.parse(line); //(Line 42 of Firesheepworker.js) as the input is not escaped.

[onuisx]

Fix also the detection of firesheep by other tools (s ee https://github.com/codebutler/firesheep/issues/117 )