Extract tmpdir into its own object
Extract file-downloading into the tmp dir

Downloads files separately from the install recipe itself and the 
installs-files step now just initiates the download and does a copy.

Because most of our steps are going to need to do intermediate 
processing on items after they've been downloaded, separating these
behaviors seems preferable.

Also, I'd rather have it download to a separate place rather than have
a failed download clobber an existing file (I once on a slow connection
saw gitx just slowly pull in hunks of ember. Neat but error-prone)
spelling
Implements "archive" recipes. 

Fixes #1

* Renames downloads-file to `downloads-files` to be more consistent
* adds new `unpacks-archives` library
* supports simple 1:1 src+dest copy declarations as well as array src's
to single destinations
* does not support any wildcards or regex
* uses decompress library for unzipping/untarring
* tmp now provides an arbitrary subpath when desired
* drops ncp in favor of cpr, which requires we branch on single-file
copies