Merge pull request protocolbuffers#915 from pherl/master

add default_runtime attribute to cc|py rules.

Merge pull request protocolbuffers#796 from jcanizales/add-missing-comma

Fix podspec by adding missing comma

Protocol Buffers v3.0.0-alpha-4

Merge pull request protocolbuffers#759 from TeBoring/beta-1-2

Fix Protobuf.podspec

bump the runtime version to 3.1.

This matches the protoc artifacts.

Change-Id: Iad8ffed6d0c61b79893a36ccafb420e79aca8d14

Modify change log

A few tweaks to CHANGES.txt.

Protcol Buffers v3.0.0-alpha-1

Protocol Buffers v2.6.1

Update CHANGES.txt to include a bug fix.