Fix Storage interface

- Method getItem returns string.
- It's possible to use property accessors to handle values in storage
  but this is not the part of standard. Also there can be confusion
  because any object can be written into property of storage object
  but when property is read string value is always returned.

https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Storage
Introduce null-friendly Storage wrapper

Null-returning methods are replaced with option-retuning ones.
Merge pull request #67 from andrey-fomin/fix-storage-interface

Fix Storage interface and introduce null-friendly wrapper