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.

package.json

@@ -6,12 +6,12 @@
     "test": "test"
   },
   "scripts": {
-    "test": "urchin test",
-    "test/fast": "urchin -f test/fast",
-    "test/slow": "urchin -f test/slow",
-    "test/install_script": "urchin -f test/install_script",
-    "test/installation": "urchin -f test/installation",
-    "test/sourcing": "urchin -f test/sourcing"
+    "test": "shell=$(basename -- $(ps -o comm= $(ps -o ppid= -p $PPID)) | sed 's/^-//'); make test-$shell",
+    "test/fast": "shell=$(basename -- $(ps -o comm= $(ps -o ppid= -p $PPID)) | sed 's/^-//'); make TEST_SUITE=fast test-$shell",
+    "test/slow": "shell=$(basename -- $(ps -o comm= $(ps -o ppid= -p $PPID)) | sed 's/^-//'); make TEST_SUITE=slow test-$shell",
+    "test/install_script": "shell=$(basename -- $(ps -o comm= $(ps -o ppid= -p $PPID)) | sed 's/^-//'); make TEST_SUITE=install_script test-$shell",
+    "test/installation": "shell=$(basename -- $(ps -o comm= $(ps -o ppid= -p $PPID)) | sed 's/^-//'); make TEST_SUITE=installation test-$shell",
+    "test/sourcing": "shell=$(basename -- $(ps -o comm= $(ps -o ppid= -p $PPID)) | sed 's/^-//'); make TEST_SUITE=sourcing test-$shell"
   },
   "repository": {
     "type": "git",
@@ -30,7 +30,8 @@
   },
   "homepage": "https://github.com/creationix/nvm",
   "devDependencies": {
+    "replace": "~0.3.0",
+    "semver": "~4.1.0",
     "urchin": "~0.0.2"
   }
 }
-