Add backend to output JavaScript

This backend supports array-based and switch-based lookup functions. It
doesn't support struct-type and duplicates at the moment.

Current version is still very C-oriented and work is required to make
this more language-agnostic.

Documentation is not up-to-date yet.
Use a if, not a switch, in the hash() function

With some browsers, switch are terrible performance wise.
Add an option to disable the in_word_set function generation
Use a "class" to enclose functions

Before, the generated script would look like:
  var hash = function() {
    ...
  }

Now, it looks like this:
  var Perfect_Hash = {
    hash: function() {
      ...
    },
  };

The name of the class can be specified using the "-Z" flag. The default
is Perfect_Hash.
Fix JavaScript syntax spotted by JSLint
Update tests/test-6.exp to match changes to "gperf --help"
doc/Makefile.in: Fix build and install when builddir != srcdir