Rename 'stashes' method to 'stash_list'

* R/stash.r: Rename Rename 'stashes' method to 'stash_list' to have
  consistent naming of methods.

* Updated documentation, tests and NAMESPACE

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Refactoring of 'stash_drop' method

* R/stash.r (stash_drop): Add index to signature of stash_drop and
  have methods for 'missing', 'integer' and 'numeric' type of index.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>