[iOS] Add support to change path directory.

Options:
 - docs: Documents (Default, if no set)
 - lib: Library
 - caches: Caches

Example:
var db = window.sqlitePlugin.openDatabase({name: "DB", path: "docs"});
var db = window.sqlitePlugin.openDatabase({name: "DB", path: "lib"});
var db = window.sqlitePlugin.openDatabase({name: "DB", path: "caches"});