API: Yahoo option type labels

[davidastephens]

#244 changed how option types are described from put and call to puts and calls.

This is a breaking API change and is different than how the google option reader describes them.
Docs broken here

Any objection to changing this back to put and call? Semantically, as the index is referring to an individual option, I think the singular is more appropriate.

Example:

In [1]: 
from pandas_datareader import Options
tsla = Options('tsla', 'yahoo')
data = tsla.get_all_data()
data.index.levels[2]

Out[1]: 
Index(['calls', 'puts'], dtype='object', name='Type')

In [2]:
tsla = Options('tsla', 'google')
data = tsla.get_options_data(expiry=tsla.expiry_dates[0])
data.index.levels[2]
Out[2]:
Index(['call', 'put'], dtype='object', name='Type')

[femtotrader]

Pinging @beluga9

[beluga9]

Sorry about that. No objections @davidastephens.

[davidastephens]

No prob. Thanks, will modify and add tests to ensure consistency.