Added support for plain 'Infinity' when feature ALLOW_NON_NUMERIC_NUMBERS is on.

This fixes a bug wherein converting a Node to string and then back fails, because
.toString() writes Double.POSITIVE_INFINITY as Infinity, but the current functionality
only supports +Infinity.
Merge pull request #84 from ebrevdo/master

Added support for parsing 'Infinity' when feature ALLOW_NON_NUMERIC_NUMBERS is on.