Fixed Pow installation on Yosemite

This is fix of basecamp/pow#452 based on @koenpunt's PR #453.

### How to install:
```sh
curl https://raw.githubusercontent.com/asux/pow/master/install.sh | VERSION=0.4.4-pre ARCHIVE_URL=https://github.com/asux/pow/releases/download/v0.4.4-pre/0.4.4-pre.tar.gz sh
```

### How to build
```sh
brew update
brew install node
git clone git@github.com:asux/pow.git ~/Projects/pow
cd ~/Projects/pow
npm --global install
./build
./install
```
You should revert 4f4a031 before release of new tarball.
**UPDATE**: added `npm install`. [comment](https://github.com/basecamp/pow/pull/465#issuecomment-55776812)
**UPDATE2**: `/usr/local/share/npm/bin` must be in your `PATH` environment variable before build [comment](https://github.com/basecamp/pow/pull/465#issuecomment-56843243)
**UPDATE3**: use `--global` option. [comment](https://github.com/basecamp/pow/pull/465#issuecomment-56847024)
**UPDATE4**: added one-line installer [comment](https://github.com/basecamp/pow/pull/465#issuecomment-58158903)