BUG: Don't call np.roll on empty arrays.
On some versions of numpy (such as 1.7, but not 1.8), this throws a
ZeroDivisionError.

Fixes #8019.

add a note in v0.15.0 to bug fixes (with the issue number)
 Updated.
 perfect, ping on green.
 OK, this should be good to go. I'm seeing consistent failures of TestYahooOptions, an external network test. I don't think this is related to these changes.
 ok, pls rebase on master and ping when green (the other options issue is fixed)
 @jreback it's green!
 @Wilfred gr8 thanks!
 someone pointed me to your package: https://github.com/ahlmss/mdf

neat!
