BUG/COMPAT: io.common.get_filepath_or_buffer should use pass encoding parameter
Ran into this when downloading a file from s3 with py3.

[This line](https://github.com/pydata/pandas/blob/master/pandas/io/common.py#L157) needs to pass the encoding parameter to the boto method `.get_contents_as_string`, else it returns a stream of bytes which trips up StringIO on python 3.

Simple fix. Will try to get to it tonight unless someone beats me to it.
