Track code coverage based on absolute thresholds.

Taking advantage of new Istanbul feature, we can guard against coverage
regression by specifying maximum uncovered entities, instead of
percentage-based thresholds.

See gotwarlost/istanbul#25 for details.

http://code.google.com/p/esprima/issues/detail?id=388

package.json

@@ -26,15 +26,15 @@
     }],
     "devDependencies": {
         "jslint": "~0.1.9",
-        "istanbul": "~0.1.22",
+        "istanbul": "~0.1.27",
         "complexity-report": "~0.6.1"
     },
     "scripts": {
         "test": "npm run-script lint && node test/run.js && npm run-script coverage && npm run-script complexity",
         "lint": "node_modules/jslint/bin/jslint.js esprima.js",
         "coverage": "npm run-script analyze-coverage && npm run-script check-coverage",
         "analyze-coverage": "node node_modules/istanbul/lib/cli.js cover test/runner.js",
-        "check-coverage": "node node_modules/istanbul/lib/cli.js check-coverage --statement 99.72 --branch 98.49 --function 100",
+        "check-coverage": "node node_modules/istanbul/lib/cli.js check-coverage --statement -5 --branch -17 --function 100",
         "complexity": "node node_modules/complexity-report/src/cli.js --maxcc 30 --silent -l -w esprima.js",
         "benchmark": "node test/benchmarks.js",
         "benchmark-quick": "node test/benchmarks.js quick"