ENH hdf write accepts attrs dict for group attribtes

`_write_to_group` now accepts `attrs=dict(...)` storing values as group attributes. This is also available for `to_hdf`, `append` and `put` to easily store metadata in the hdf file:
```
df.to_hdf('df', attrs=dict(source='some-file.csv', processed='software-v0.42.rc3'))
```