initial work on own package skeleton

NOTE: not ready for merge; ready for comments.

This implements a trimmed-down version of `package.skeleton()` which creates a package which should pass `R CMD check`.

There are a couple things we have to make a decision on:

```
* checking DESCRIPTION meta-information ... WARNING
Non-standard license specification:
  What license is it under?
Standardizable: FALSE
```

Do we want a default license?

```
* checking for missing documentation entries ... WARNING
Undocumented code objects:
  ‘hello’
```

Should we include an example `.Rd` file for our automatic 'hello_world' function? (Do we want a hello world function?) Any other tweaks we could make to the `DESCRIPTION` file -- e.g. respect any `devtools.*` options?
I think it's overstepping to insert a default license so let's leave as-is