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ARROW-1025: [Website] Improved changelog for website, include git shortlog

This patch combines new features and improvements in the changelog into a single category and lists these first on the website (since these may be more interesting for readers). I also added the `git shortlog` to show who contributed to each release

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #691 from wesm/ARROW-1025 and squashes the following commits:

416c876 [Wes McKinney] Consolidated changelog for the website with new features/improvements shown first. Include git shortlog