Added support of LCOV.

* .gitignore: Ignored LCOV source trees.
* bootstrap: Added support of `--enable-lcov` option.
* jamroot: Added support of LCOV.
* lcov/jamfile: Newly added.
* lcov/latest.sh: Newly added.
Silenced `lcov/latest.sh`.
Merge pull request #42 from Cryolite/feature/lcov

feature/lcov