Issue #282 Gist Comment test implementation.

Hello, 

PLEASE DO NOT MERGE THIS! 
It is rather an approach-offer. 

A) Sorry for delaying this. 
B) Unfortunately this task went a bit too complicated for my understnanding of PDD and current architecture. 

So, the issues which are there: 

1. Think we need an object for an Asset abstraction, which we don't have now. 
1. Thus, need to make all the necessary adjustments to accomodate for it (mock, etc). 
1. Bigger issue. What I see now is that `Request` object bearing base url migrates from object to object. This leads me to the idea the lib may be used not only with github.com but also with the hosted githubs. Now, the issue comes in: the rest of the calls are api calls, and are issued towards single well-defined URL (base one). For the upload-assert call, even Github uses different upload endpoint. How do we handle this? http://developer.github.com/v3/repos/releases/#upload-a-release-asset


I know this is quite huge, and sorry for delaying it. I will discuss technical issues with Yegor. 

Alex.