Code changes:
 - Removing leading_zero_possible from the metadata and all the places it is
   referenced in the build and prod code. Will be removed from the metadata
   proto itself in a subsequent release. This should not affect users of the
   library - the only place it was used was FormatInOriginalFormat, and only
   initially to try and avoid modifying the input number by removing/adding
   digits inadvertently. Now this is checked at the end of the method anyway.
   However slight formatting differences with this method on invalid numbers
   starting with 0s may be noticed in some countries.
 - Bug fix for Javascript: getNationalSignificantNumber used to print the string
   "null" for empty phone number objects, now it returns an empty string.
 - Updated the documentation for the isNumberGeographical API.
 - Small comment improvements for parse() method to point users at
   parseAndKeepRawInput().
 - Added a new enum for CountryCodeSource called UNSPECIFIED. This is used as a
   default value, and will be returned if someone calls parse() and then
   getCountryCodeSource() on the result. If users want an actual value for this
   then they should call parseAndKeepRawInput() instead; the values that were
   previously returned after calling this method will not change.