ENH: add arguments for scatter_plot

Modifies pandas/tools/plotting.py

* xlim, ylim, sharex, sharey, xlabelsize and ylabelsize arguments are added
* To handle the common arguments for axes decoration, _decorate_axes() function has been added (currently, only scatter_plot uses this function).
* kwds argument added which is passed to ax.scatter()
* Some test cases are added.

This pull request should be merged to following pull request, but I've mistaken to merget the latest master branch.
https://github.com/pydata/pandas/pull/1947