Activate cache by default on BaseQuery.
Improved Eso.get_header()

Accepts an astropy.table.Column as input.
Returns an satrapy.table.Table.
Replaced "/".join() by os.path.join()

in BaseQuery and EsoClass
Renamed Eso.get_header() to Eso.get_headers()

Since we can get more than one header at a time.
Extended Eso documentation with get_headers().
Typo and documentation clarification.
Merge pull request #302 from jwoillez/cache-in-config

Caching turned on by default, improvements to Eso.get_header()