pour liquids into containers on ground

Pour some liquid from inventory to near tile with *containers*.

* Old behaviour: liquid pours on ground.

* New behaviour:
![pour](https://cloud.githubusercontent.com/assets/6351438/3560592/a6fd3758-0978-11e4-88db-05ca3879d187.png)


There's a method that does almost exactly what you want already, inv_for_liquid( item  &liquid, std::string title, bool autopickup)
Like with the eat from ground PR, you can use that code as a template, but just use form_from_ground() to create the inventory to operate on.