feat(*): update to latest gRPC and Protobuf

Add Git and Mercurial to build prerequisites

Closes: #2343

fix(tiller): update tls client auth policy

Effectively, this switches us to gRPC's internal version 4. This is
compatible with protoc 3.2 and grpc-go 1.2.1.

Update Slack channels

Improve get script

Added reference to plugins

ref(helm): use new debug method to standardize debug output

RequireAndVerifyClientCert => VerifyClientCertIfGiven

Closes #2310

chore(*): bump kubernetes to v1.6.2

chore(README.md): update zoom meeting link

ref(*): Regenerates protobuf files with protoc 3.2.0

Protoc was updated a little while ago and we are likely to get more
people using it. This should avoid weird merge conflicts and bump us
to the latest version

feat(tiller): add --net-host flag to 'helm init'

test(*): add tests for new tls support

Fix indent for defaultDeployment

Search constraint and Install/Upgrade default constraint

Validates chart-dir before running dependency list command

When 'helm init --net-host=true' is run, Tiller will be installed with access to the host network (net=host in Docker)

Closes #2222

Adds a testdata directory to hold tls certs at the root
of the project. The tests cover pkg/tlsutil, cmd/helm,
and cmd/helm/installer.

Closes #2289

Implementation of pluggable downloaders

Fixes issues/2276: helm dependency list hangs if run on large directory

It is now possible to create plugins with chart download capabilities for custom, non-http protocols.
Furthermore it is possible to reuse helm packages to implement alternative clients with these custom downloader functions.

Exposes repository-URLs as flags on helm init command

Expose tiller server metrics to prometheus