feat(autocomplete): Add support for $http promises

Add support for $http promises so they can be returned as is by the
source option expression without requiring its value to be converted.

Closes #38.
test(autocomplete): Add missing tests

Add missing tests covering propagation handling of non-hotkeys keystrokes.
refactor(autosize): Remove redundant placeholder check

Remove redundant placeholder check for undefinedness since it'll
always have a value by definition.