feat(plugins): add getLastRelease hook

The code to determine the last-release is now in its own repository: https://github.com/semantic-release/last-release-npm
One can overwrite that behavior by defining a "getLastRelease" plugin. This way
one can decouple semantic-release from npm, e.g. by implementing a git-tag
based version. This is already worked on: https://github.com/semantic-release/last-release-git-tag

Closes #56

package.json

@@ -24,12 +24,12 @@
     "@semantic-release/commit-analyzer": "^2.0.0",
     "@semantic-release/condition-travis": "^4.0.0",
     "@semantic-release/error": "^1.0.0",
+    "@semantic-release/last-release-npm": "^1.1.1",
     "@semantic-release/release-notes-generator": "^2.0.0",
     "git-head": "^1.2.1",
     "github": "^0.2.4",
     "lodash": "^3.9.3",
     "nopt": "^3.0.3",
-    "npm-registry-client": "^6.4.0",
     "npmconf": "^2.1.2",
     "npmlog": "^1.2.1",
     "parse-github-repo-url": "^1.0.0",