- cache the nvm cache on travis

 - 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

Many makefile enhancements (support for direct invocation, instructions if invoked without target, release mechanism improved), package.json scripts now invoke the makefile.

- Using `npm test` and `npm run …` scripts defined in package.json now invokes the makefile and runs the test with the same shell that npm was invoked from.
- The makefile can now be invoked directly - supporting utilities from locally installed npm packages are automatically discovered.
- Invoking the makefile without a target errors out with a hint.
- Shell-specific test targets are now named 'test-<shell>'.
- Both 'test-<shell>' targets and the all-shells 'test' target now run all test suites by default.
- On `make TAG=<new-version> release` there must be no uncommitted changes. '<new-version>' can now also be one of the following increment specifiers: 'patch', 'minor', 'major'.
- It is ensure that <new-version>, if not an increment specifier, is a valid semver version number that is higher than the previous release's.
- The previous release tag is now located with a pattern so as to exclude tags that aren't version numbers.
- Switched from lightweight to annotated tags for releases.

fast tests

alias

unalias

more alias tests

tests

document testing

warn not to run while testing

chmod +x deactivate

specify the version (commit) of urchin

installation test

run

tests for ls

switch unsetopt to the thing creationix has

rename tests

urchin log

adjust urchin version

Aliases are stored as plaintext files in the $NVM_DIR/alias dir.
They may store either an explicit version (v0.3.6) or an implied version
("latest"). The latter is a "moving target", and thus possibly
dangerous, but can be useful, too.