- fixed Xcode 8 warnings.

- fixed a bug where the task was not cancelled and the downloaded ima…

…ge in a list was set more then once.

changed to version 1.0.5

- fixed notification bug where addObserver's object property was self…

… instead nil.

- changed urlString to be nullable so it doesn't required nil check w…

…ith Swift.

- added __nullable and __nonnull type annotations for the headers.

- added LICENSE.md file