Issue 794: initial commit

Created pull request for the https://github.com/jcabi/jcabi-github/issues/794 . Please review it.
@carlosmiranda I use quotes in xpath of MkTree. If I just remove append qotes it will throw exception like 

com.jcabi.xml.ListWrapper$NodeNotFoundException: XPath '/github/repos/repo[@coords = 

'jeff/test']/git/trees/tree[sha = 'abcsha12']' not found in '<?xml version=\"1.0\" encoding=\"UTF-

8\" standalone=\"n..273..fs>\r\n</git>\r\n</repo>\r\n</repos>\r\n</github>\r\n': Index (0) is out of 

bounds (size=0)
	at com.jcabi.xml.ListWrapper.get(ListWrapper.java:153)
	at com.jcabi.github.mock.MkTree.json(MkTree.java:97)
	at com.jcabi.github.mock.MkTreeTest.fetchesContent(MkTreeTest.java:54)