add makefile release task

Added simple release task to `Makefile`, replaces occurrences of last git tag (`git describe --abbrev=0 --tags`) in `README.markdown` and `install.sh` with supplied `TAG` variable and tags the repo with supplied tag:

```sh
make release TAG=v0.11.0
```
The version is not specified in `nvm.sh`. Or is that not what you mean?