DOC: Fixed outdated doc-string, added missing default values, added missing optional parameters for some io classes
Some doc-strings are outdated, in a sense that there are missing newly added optional parameters and default values. 

@terrytangyuan The changes look good, but, the `data.py` is now maintained at https://github.com/pydata/pandas-datareader. So maybe you can do a PR for that file there
 @jorisvandenbossche Nice catch! Thanks for checking. I've made the changes. I'll submit another PR to make changes on the new file. 
 @jorisvandenbossche On green now. Can you merge? 
 Thanks @kawochen Fixed and on green now. 
 I guess this is fine. @terrytangyuan prob strip off the stuff for `pandas-datareader` and submit there as well.
