Stringify process

Why bother about this? A `CAF::Process` is supposed to be an object. What's the advantage of

```perl
if (CAF::Process->is_executable($foo)) {
    my $cmd = CAF::Process->new([$foo], log => $self);
    ....
```
vs

```perl
my $proc = CAF::Process->new([$foo], log => $self);
if ($proc->is_executable()) {
   ...
```
you allow both, and I'd rather see only one idiom. The second one looks cleaner to me, and it leads to less code in your implementation.