Merge pull request ethereum#3950 from ethereum/develop

Merge develop into release for 0.4.23

Merge pull request ethereum#3892 from ethereum/develop

Merge develop into release for 0.4.22

Merge pull request ethereum#3678 from ethereum/develop

Merge develop into release.

Merge pull request ethereum#3503 from ethereum/develop

Merge develop into release for v0.4.20.

Merge pull request ethereum#3261 from ethereum/develop

Merge develop into release for 0.4.19

Merge pull request ethereum#3099 from ethereum/develop

Merge develop into release for 0.4.18.

Merge pull request ethereum#2947 from ethereum/develop

Merge develop into release for 0.4.17.

Merge pull request ethereum#2802 from ethereum/develop

Merge develop into release for 0.4.16

Merge pull request ethereum#2709 from ethereum/updateChangelog

Update changelog.

Merge pull request ethereum#2667 from ethereum/develop

Merge develop into release in proparation for 0.4.14