Fixed floatval() and setlocale() issue & escaping-issue for json_decode()

### Issue 1:

The use of [floatval()](http://php.net/manual/en/function.floatval.php) in PHP depends on the locale ([setlocale()](http://php.net/manual/en/function.setlocale.php)).

With `setlocale(LC_NUMERIC, 'en_US');` a `var_dump(floatval('0.26'));` will output:

    float(0.26)

But with e.g. `setlocale(LC_NUMERIC, 'de_DE');` it will output:

    float(0,26)

This causes some Vis (in specific Zoom and Smoothie) to break, because they can only handle floats with dots.

A small fix is the str_replace, but in general I think it would be better to set only the `LC_NUMERIC` always to en_US in locale.php.

### Issue 2:

When editing values and e.g. the URL `dashboard/set.json?id=1&fields={"alias":"test"}` is called, `$_GET['fields']` will be on some PHP-settings escaped. So this will be called:

    json_decode('{\"alias\":\"test\"');

Because thats not a valid JSON-string, json_decode will return `NULL` and the SQL-query will fail. To fix this, I added [stripslashes()](http://php.net/manual/en/function.stripslashes.php) everywhere it's needed.