RemoteMap: add 'get' method

I often stumble on this one, as (I think) it's a common pattern to get a user specified option variable with a default value, for instance:
```
 if self.vim.vars.get('myplugin_some_bool_option'): #defaults to None (which is false)
    # do stuff,

```
The more fullblown alternative would be to use `UserDict.DictMixin`, if one also wants to support iterators etc, but I think this is the most useful `dict`-method for the intented usage.