tools/mkrelease: Add a get_project_root() function

Issue #116.
tools/mkrelease: Add get_{major,minor,micro}() functions

Issue #116.
tools/mkrelease: Add get_current_version() function

Issue #116.
tools/mkrelease: add a verify_version() function

Issue #116.
tools/mkrelease: Sanity-check the given version

Issue #116.
tools/mkrelease: Add replace_version() function

Issue #116.
TMP