Add REST endpoint for contribution statistic: https://bugzilla.redhat.com/show_bug.cgi?id=1092193

Squashed commit of the following:

commit 4814c6cfae6b85045f43a6438b305a995ec5d69a
Author: Alex Eng <aeng@redhat.com>
Date:   Thu Oct 9 14:18:38 2014 +1000

    Add date-format for daterange

commit e9754f79e809fc21233487fe6377563b6663538a
Author: Alex Eng <aeng@redhat.com>
Date:   Thu Oct 9 13:29:04 2014 +1000

    Add unit test for ContributionStatistics

commit 977780e3cea7cbd44af27782069f25052075ea1c
Author: Alex Eng <aeng@redhat.com>
Date:   Thu Oct 9 13:11:43 2014 +1000

    Add REST endpoint for contribution statistic

commit ae4a5773d257521c1d1bda594f6b1d720d0bf202
Author: Alex Eng <aeng@redhat.com>
Date:   Thu Oct 9 10:57:55 2014 +1000

    Create dto for contribution statistic