Implementation of pluggable downloaders

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

Fixes issues/2272

stable-repository-url is currently hard-coded for helm init,
so if a user wants to make a different intranet repository as stable repo,
only option left is to first do an init,
followed by repo delete and repo add.

chartutil create ingress example

clarify helm serve's intent

fix(tiller): ignore empty YAML documents during delete

A lot of community members are looking at `helm serve` as a production webserver to host their
charts. There are much better solutions out there like s3, wabs or gcs for cloud storage and
nginx, haproxy or apache for DIY dedicated web servers. Steering users in the right direction
will help users make the right decision when it comes to serving their charts in production.

Closes #2256

fix(helm): fix nil pointer in requirements.go

fix(scripts): don't include plugins for generating docs

docs(*): Add ci script to confirm docs are up to date with commits

This fixes a segfault that was the result of assigning to a property of
a nil pointer.

Closes #2244

* tiller_fix_2261

* fix(tiller) 2261 Increase maxMsgSize to 20MB

fix(helm): correct import for apps api

* cleanup named imports

ref(*): kubernetes v1.6

feat(tiller): support version constraint on chart

fix(helm): remove duplicate commands

docs(related): add note about helm-plugin GitHub tag

Rebase on kubernetes v1.6

fix(init): use ImagePullPolicy Always for canary installs

fixes: #2155

Added release-name to helm delete error

Fixes https://github.com/kubernetes/helm/issues/2251
In case a user tries to delete a non-existent release,
the error message displayed was missing release-name.

Add note about gopath in developer docs

Fixes hard-coded linux based file-separator

Fixes https://github.com/kubernetes/helm/issues/2254

Added a note that helm needs to be in a
specific directory path in order for it
to be compiled.

ref(plugin): cleanup unused functions