Add task exports, default version for -d, and pod metadata

- Namespaces packaged in jars in maven may now declare that they export
  boot tasks. This makes those tasks usable from the command line boot
  via boot -d.

  Eg.
      (ns ^:boot/export-tasks my.task.namespace
        ...)
  or
      (ns my.task.namespace
        {:boot/export-tasks true}
        ...)

- When adding dependencies on the command line via -d boot option, the
  version is now optional. When not provided the latest release version
  is assumed.

- The pod functions boot.pod/eval-in, boot.pod/call-in, etc. now support
  passing metadata between environments. Metadata is enabled by binding
  *print-meta* to true in the calling environment. Note that metadata
  must be printable and readable or exceptions will be thrown.

  Ex.
      (meta (binding [*print-meta* true]
             (boot.pod/eval-in my-pod
              (with-meta [1 2 3] {:foo :bar})))
      => {:foo :bar}