Fixed #96.

* bootstrap:
  * Modified to dereference `latest` value of `--enable-lcov` command-line
    option into concrete version numbers before invoking `bjam`.
  * Modified to prepare LCOV source trees before invoking `bjam`.
* jamroot: Removed support for `latest` value of `--enable-lcov`
           command-line option.
* lcov/checkout.sh: Newly added.
* lcov/latest.sh: Fixed to use git repository.
* lcov/download.sh: Newly added.
* lcov/jamfile:
  * Removed downloading and expanding actions.
  * Switched dependency on LCOV source trees from `lcov-$VERSION/README`
    to timestamp files `lcov-$VERSION/e2c10538-d490-4c93-8e19-3fb74e470174`.