Simplify getting contributors information

[mockitoguy]

Problem

Now we have 2 ways of getting contributors and it adds some complexity to the code. During the build we use both ways of getting contributors information, wrapped in separate Gradle tasks. Let's have one robust service for getting contributors and one Gradle task

Workflow

1. First, we hit repo/contributors endpoint and get all of the guys (https://developer.github.com/v3/repos/#list-contributors).

• Let’s use max page size :D

2. Since we may not get the most recent contributors due to GitHub performance limitations, let’s do another hit for commits (https://developer.github.com/v3/repos/commits/#list-commits-on-a-repository).

• Let’s limit the commits to last 24hrs, earlier commits should be covered by repo/contributors request

3. We combine the results

Implementation

1. Update GitHubAllContributorsFetcher. @mstachniuk, can you take a stab at it at some point? :)
2. Rework Gradle tasks, eliminate dead code - I'll do it with pleasure! (@szczepiq)