Don't throw TypeError when checking for equality to primitive types

This fixes a bug which makes an equality check like below raise a ``TypeError``.

```pycon
>>> from orderedmultidict import omdict as om
>>> om({'foo': om()})
omdict([('foo', omdict([]))])
>>> om({'foo': om()}) == 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\repos\github\denizdogan\orderedmultidict\orderedmultidict\orderedmultidict.py", line 716, in __eq__
    elif len(self) != len(other):
TypeError: object of type 'int' has no len()
```