Fixed #109.

* bootstrap:
  * Modified to dereference `latest` value of `--with-gcovr` command-line
    option into concrete version numbers before invoking `bjam`.
  * Modified to prepare gcovr source trees before invoking `bjam`.
* jamroot: Removed support for `latest` value of `--enable-gcovr`
           command-line option.
* gcovr/checkout.sh: Newly added.
* gcovr/latest.sh: Fixed to use git repository.
* gcovr/download.sh: Newly added.
* gcovr/jamfile:
  * Removed downloading and expanding actions.
  * Switched dependency on gcovr source trees from
    `gcovr-$VERSION/README.txt` to timestamp files
    `gcovr-$VERSION/fb301e39-8f74-4e35-8428-9b3b48a324bc`.