Small parser optimizations

This changes the `parseLengthCodedNumber` and `parseUnsignedNumber` methods on the parser to use more optimized versions. This replaces `Math.pow` calls with faster bitwise operations, adds more tests for these parsers, simplifies the detection of numbers >= 53 bits, and even includes the actual >= 53 bit value in the error message when big number support is disabled.

Simple jsperf for the pow -> bitwise change: http://jsperf.com/parse-uint/3