Allow adding directories via curl

Version/Platform/Processor information (from `ipfs version --all`):

go-ipfs version: 0.4.3-dev-b5fbc01
Repo version: 4
System version: amd64/linux
Golang version: go1.6.2

Type (bug, feature, meta, test failure, question):

feature

Area (api, commands, daemon, fuse, etc):

api

Priority (from P0: functioning, to P4: operations on fire):

Description:

Provide a command, which takes a tarball, untars it and adds the untared content to ipfs.
This is useful to be able to add directories to ipfs via curl.