Fix "TypeError: Arguments to path.resolve must be strings" error when using node v0.10.0.

Using `jam install` with node v0.10.0 causes the following error:

    Error: TypeError: Arguments to path.resolve must be strings
        at Object.exports.resolve (path.js:313:15)
        at Object.exports.extendOptions (/usr/local/share/npm/lib/node_modules/jamjs/lib/commands/install.js:133:32)
        at /usr/local/share/npm/lib/node_modules/jamjs/lib/commands/install.js:97:23
        at /usr/local/share/npm/lib/node_modules/jamjs/lib/commands/install.js:301:13
        at /usr/local/share/npm/lib/node_modules/jamjs/node_modules/async/lib/async.js:677:28
        at /usr/local/share/npm/lib/node_modules/jamjs/lib/project.js:74:20
        at /usr/local/share/npm/lib/node_modules/jamjs/lib/project.js:47:20
        at Object.cb [as oncomplete] (fs.js:168:19)

This is fixed by falling back to the project directory if `settings.baseUrl` is undefined.