Added options for customize default error messages

The text for handling errors like file exceeding maxFileSize where harcoded in the JS and it can't be changed.

Now, when you initialize the fileupload, you can provide a custom message for each restriction.

E.g.

```javascript
  $('#photos_area').fileupload(
    maxNumberOfFiles: 10,
    maxFileSize: 3000000, // 3 MB
    maxFileSizeError: "File size exceeds the maximum allowed size (3 MB)",
    acceptFileTypes: /(\.|\/)(jpe?g)$/i,
    acceptFileTypesError: "Invalid file format. Only jpeg files are allowed"
  )
```
If you want more customization, the errors can also be functions that receives the file that triggered the error and the options of the fileupload

```javascript
  $('#photos_area').fileupload(
    maxFileSize: 3000000, // 3 MB
    maxFileSizeError: function(file, options) {
      file.name + " size exceeds the maximum allowed size (" + options.maxFileSize / 1000000 + "MB)"
    }
  )
```

You can personalize `maxFileSizeError`, `minFileSizeError` and `acceptFileTypesError`, but is trivial to add more messages.

If you don't provide a custom message error, the error messages are the plain old messages ("File is too big", "File is too small" and "Filetype not allowed")