Simpler (tested) parsers instead of field classes

Field objects were not doing anything apart from the parse function,
so just defining them as a bunch of functions called parsers. Saber
exports it as `Saber.Parser` object. A parser will be
`Saber.Parser.<type>`. Also there is Saber.Parser.identity which is
used by default for unavailable parsers.

To define a new parser:

Saber.Parser.newtype = function (value, typeData) {
  // `value` will be the actual value to be parsed and
  // `typeData` can be a object which can be used to
  // configure the parser. For eg: `integer` provides
  // `radix` option to set the radix that is used with
  // parseInt function.
};

Signed-off-by: Rohan Jain <crodjer@gmail.com>