Update SkipList. Fixed Issues #137, #138, #139, #140

Fixed #137 . This bug occurs when _getNextLevel() method returned 0.

Fixed #138 and #139 . This bug occurs because of skip list was сircular.

Fixed #140. This bug occurs comparing method was called for null object.

I add implementaion for public T this[T item]. I think this should be work like a standard Dictionary type.

I divided a unit test for SkipList on some tests. And added additinal tests.

[x] An issues were first created before opening this pull request
[x] The new code follows the contribution guidelines
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have made corresponding changes to the documentation
[x] My changes generate no new warnings
[x] I have added tests to ensure that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes

aalhour / C-Sharp-Algorithms