Adds Command Line Interface for generating PNGs from mermaid description files

As discussed in #6, the following PR adds a CLI for mermaid which is exposed as a `mermaid` command when installed globally, via `npm install -g mermaid`. `mermaid` will look for the `phantomjs` command in your *PATH* and give helpful errors if it's not found, or is too old. Additionally, you can specify the path to phantomjs with the `-e` switch.

Otherwise, functionality is identical to my [mermaid-cli](https://www.npmjs.com/package/mermaid-cli) package, which will be deprecated once this PR is accepted and a new version including the CLI is published. Then, we can focus on improvements here!