Create dispatcher from object instance or dictionary.

I extend dispatcher class with optional initial mapping. I might be used in the way below:

```python
In [1]: from jsonrpc.dispatcher import Dispatcher

In [2]: class Dummy(): 
   ...:     def one(self):
   ...:         pass
   ...:     

In [3]: Dispatcher(Dummy())
Out[3]: <jsonrpc.dispatcher.Dispatcher at 0x7f246c504050>

In [4]: dummy = {
   ...:         'one': lambda x: x,
   ...:         'two': lambda x: x,
   ...: }

In [5]: Dispatcher(dummy)
Out[5]: <jsonrpc.dispatcher.Dispatcher at 0x7f246c5040d0>

```