compiler: resolve paths in middleware params

Introduce a convention for specifying relative paths in middleware
params: values prefixed with `$!` and starting with `./` or `../`
are resolved relatively to `middleware.json`.

Example:

    {
      "files": {
        "loopback#static": {
          "params": "$!../client"
        },
        "loopback#static": {
          "params": "$!./public"
        }
      }
    }
Merge pull request #74 from strongloop/feature/relative-paths-in-middleware-params

compiler: resolve paths in middleware params