Changes for Github API from v2 to v3

Github API v2 was deprecated and removed so plugin doesn't work.
I rewrite iessues' fetcher to use API v3.

P.S. I use Jakson for JSON and put it to lib folder but TeamCity doesn't use them in plugin folder so I put them into TeamCity WEB-INF/lib folder. But I don't like this hack... Do you have some advice to help with this issue?