Merge pull request ethereum#74 from ethereum/dev

v0.2.1 Release

Merge pull request ethereum#57 from ethereum/version_bump_v0_2

Bump app version from v0.1.0 -> v0.2.0 to align with release versioning

Merge pull request ethereum#43 from ethereum/dev

Merge `dev` to `master` (v0.12.x mode)

Adds preproduction warning