BUG: GroupBy.size created by TimeGrouper raises AttributeError

Related to #7453 2nd issue:
- ``GroupBy.size`` created by ``TimeGrouper`` raises ``AttributeError``
There are 2 reasons.
- ``value_counts`` cannot fulfill intermediate timestamps which is not included in the group key. For example, if the ``TimeGrouper`` categorize ``[2011-01-01, 2011-01-02, 2011-01-04]`` with daily frequency, the resulted group keys must be ``[2011-01-01, 2011-01-02, 2011-01-03, 2011-01-04]``

- ``BinGrouper`` doesn't retain original ``Index`` to be directly passed to ``value_counts``. To use ``value_counts``, it must be created using similar logic.
```
indices = self.indices
labels = []
for k, v in compat.iteritems(indices):
            labels.extend([k] * len(v))
value_counts(labels)
```