add strftime() function

Example usage:

```
body common control
{
      bundlesequence => { run };
}

bundle agent run
{
  vars:
      "time" int => now();
      "now" string => strftime("%F %T", now());
      "then" string => strftime("%F %T", 0);
  reports:
    cfengine::
      "time $(time); now $(now); then $(then)";
}
```

Output:

```
R: time 1363289351; now 2013-03-14 15:29:11; then 2013-03-14 15:29:11
```