Copies script from GNU ease.js.
* .gitignore: Do not ignore `build-aux/vergen'.
* build-aux/vergen: Add file from GNU ease.js.
* configure.ac: Use `vergen' to generate version string.
``Building'' Liza will not be required to use it; see ease.js for the style
of building that will be used here. In particular: the build process will be
used for minification and the creation of distribution archives, but one can
simply include the source files (using, say, node.js) to use it without
building.