Create in dev mode

Adds a "createInDevMode" setting, which defaults to true. When checked, packages created outside of `.atom` will be linked in to `.atom/dev/packages` instead of `.atom/packages`, which seems like a sensible place to put a package you're just writing.

See atom/atom#2148.

![screen shot 2014-05-16 at 1 54 38 pm](https://cloud.githubusercontent.com/assets/17565/3000397/3268233e-dd23-11e3-9598-0472d44d50bb.png)