Add `ckan version`, with support from the build process.
Use `git describe --long` for version numbers.
Detect certificate and other web failures, and handle accordingly.

In all cases, we remove partially downloaded files.

If we detect a certificate error, we promptthe user to use `mozroots`.

Closes #24
Added `ckan clean` to empty the cache directory.

Part of #25.
Cleanup: Replace FileSystem.IsDirectory() calls with Directory.Exists()
Find Steam on OSX.
Merge pull request #35 from johndalton/master

Clean up directory checks and find Steam on OSX.

Closes #31
Addresses #23 and friends.
Updated all files to have a consistent style.

In particular:

- MethodsLikeThis()
- attributes_like_this
- K&R braces
- Using inside namespace blocks.