Dataframe extension integration

This line currently causes an `HdfsError` (https://travis-ci.org/mtth/hdfs/jobs/30324153). Maybe try the following:
```python
try:
  from hdfs.ext.dataframe import read_df, write_df
except HdfsError:
  SKIP = True
else:
  SKIP = False
  import pandas as pd
  from pandas.util.testing import assert_frame_equal
```
Then in `setup` you can simply check for `SKIP` rather than using a separate `try`. (Note: untested!).