Javascript library should do input validation

[drinckes]

Passing values as strings instead of numbers will produce unexpected results, due to Javascript string handling.

The inputs (lat, lng, code length) should be coerced to numbers (using Number()) and if it produces NaN throw an exception.

While you're there, adding an exported constant for the code length e.g. OpenLocationCode.CODE_PRECISION_NORMAL and OpenLocationCode.CODE_PRECISION_EXTRA seems like a good idea.

[drinckes]

This got addressed in #57.