- cache the nvm cache on travis

 - `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`

 - `nvm uninstall` no longer removes source artifacts

 - consolidate `nvm_install_merged_node_binary` with `nvm_install_node_binary`
 - add `nvm_get_download_slug`, `nvm_download_artifact`
 - `nvm uninstall` no longer removes artifacts

Fixes #1216

Fixes #1212. Closes #1213.

[Tests] add `shellcheck`

 - `nvm ls-remote`
 - `nvm version-remote`
 - `nvm install`
 - `nvm uninstall`

Document existing support:
 - `nvm use`
 - `nvm exec`
 - `nvm run`

Fixes #1208.

[Refactor] `nvm install`: remove unused `REINSTALL_PACKAGES_FROM` argument to inner installation helpers.

Line 302/301 `printf "$SOURCE_STR"` is missing a trailing newline char,
while line 281/282 and 285/286 have their own new line char, so I  just
remove the manually added newline char, and add it to the end of the
variable "$SOURCE_STR", so no more manually newline char needed there.

Per https://github.com/creationix/nvm/issues/539#issuecomment-236830764