ENH: Allow for stock dividends, and in particular, Google's

recent 2 for 1 stock split, where 1 class C share was distributed
for each share of class A held.

Now a dividend can specify a sid and ratio of stock that will be paid
to owners of the original security.  If the ratio is 2.0, then for every
existing share, two shares will be paid.
DOC: Replaced "i.e." with "e.g." where appropriate in module