Fix #161: support save/load BiRNNBuilder

[danielhers]

I implemented a separate case in _save_one and _load_one for the BiRNNBuilder, but I think it might be more elegant to make it extend the Saveable interface instead. The problem with this is that the builder layer sub-components are saved in a list of tuples rather than directly as attributes, and Saveable assumes that any non-picklable sub-component is a direct attribute of the object.
A possible way to go around this is to replace the above assumption with a decorator that would declare an attribute as non-picklable (explicit is better than implicit), and add this decorator to the builder_layers attribute. The Saveable interface methods can be implemented like so:

def get_components(self):
    return [x for (f,b) in self.builder_layers for x in (f,b)]

def restore_components(self, components):
    self.builder_layers = zip(components[0::2], components[1::2])