Added support of gcovr.

* .gitignore: Ignored gcovr source trees.
* bootstrap: Added support of `--enable-gcovr` option.
* jamroot: Added support of gcovr.
* gcovr/jamfile: Newly added.
* gcovr/latest.sh: Newly added.
Merge pull request #43 from Cryolite/feature/gcovr

Added support of gcovr.