Adding a user-agent to the http getter will enable servers to
distinguish between helm (including various versions) and other
tools connecting to the server.
This flattens the getter package tree, adds tests, and changes a little
bit of the terminology to follow Go idioms. This also makes much of the
getter API private to begin with. This will give us more flexibility in
the future.
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.