driver: add internal sysfs driver iterator function
bus: add sysfs_bus:get_drivers() iterator
bus: add sysfs_bus:get_device()
bus: add sysfs_bus:get_driver()
bus: add comment for the bus object
device: comment sysfs device object
driver: add sysfs_driver:get_attributes() iterator
class: remove sysfs_class:get_devices()

Note:

This method is obsolete because it makes more sense to use the class_device
iterator function.
class: make comment for sysfs_class:get_class_devices() more clear
class: add sysfs_class:get_class_device()
common: add sysfs.path_is_link()
common: add sysfs.path_is_file()
common: add sysfs.path_is_dir()
module: add sysfs_module object and all its methods
driver: add sysfs_driver:get_module()