fix conversion issues when converting from formats with alpha channel(png,webp) to jpeg format

[Nitin-Yadav-1]

fixes #32

Converting from png/webp to jpeg creates issues as jpeg doesn't support alpha channel. Simply discarding alpha channel by 'Image.convert()' function of Pillow also creates issues with png/webp images having transparent background.

Added a fix where png/webp images are put on a new blank image of same size. This fills the transparent background and then image can be converted to jpeg. I set the fill color to be black but perhaps this can be set by user through an argument to 'ResizedImageField'.