Bug: within(directory){ execute(string) } should either 'just work' or 'boom'

docs aren't good enough to explain the design choice IMHO - just check
stackoverflow...

one solution is just to fix it - use shellwords to properly escape the command

``` ruby
require 'shellwords'

def execute(*args, &block)
if args.first.is_a?(String)
command = Shellwords.escape(args.first)
end

# ...
end
```

otherwise raise an exception

``` ruby
def execute(*args, &block)
if args.first.is_a?(String) and inside_within?
raise "don't do that"
end

# ...
end
```

the current behavior of doing

``` ruby
within directory do # silently ignored
execute command
end
```

just isn't POLS - the point of a library like cap is to be able to re-use code
but, currently, each and every use must re-invent 'cd into a (properly escaped
directory) and run commands', including handling the fact that

``` ruby
within(does_not_exist) do # raises
end
```

``` ruby
execute "#{ does_not_exist }; command.sh" # reports a failed exit status that leads to debugging which part failed
```

a final solution would be to remove the 'within' API since it sometimes works,
and sometimes does not, issuing no exception nor warning