Add `ginkgo convert command`

Usage: `ginkgo convert path/to/package`

Converts all of the *_test.go files in the package's dir recursively to
Ginkgo. Also runs ginkgo bootstrap for each package converted.
Support converting packages that only contain subpackages

Came across this testing out `ginkgo convert` against the bosh agent. There
are some packages (eg: davcli) that contain no Go files or testfiles, but
only contain more packages.
Upcase Ginkgo in the generated Describe
Move `ginkgo convert` fixtures into more canonical dir
refactor `ginkgo convert` tests