[Breaking] adapt `nvm_install_node_binary` to `nvm_install_binary`, eg, io.js too.
- `nvm_get_checksum` now accepts 5 args (from 3): flavor (node/iojs), type (binary/source), version, slug, compression (xz/gz)
- `nvm_install_binary` accepts 3 args (from 2): flavor (node/iojs), kind (std), version
- remove `nvm_install_iojs_binary`