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 https://github.com/gotwarlost/istanbul/issues/25 for details.

http://code.google.com/p/esprima/issues/detail?id=388
Run a simple script which shows the most cyclomatic-complex functions.

http://code.google.com/p/esprima/issues/detail?id=395
Reduce the cyclomatic complexity of numeric literal scanning.

Spin off the hex and octal handling to two separate functions. This
decrease the cyclomatic complexity of scanNumericLiteral from 28 to 19.

http://code.google.com/p/esprima/issues/detail?id=396