Alpha blending for better text visibility

Hello @ankush-me,

In `poisson_reconstruct.py` is the:

IM_TOP -> text instance
IM_BACK -> background image

There says:
```
combine images using poission editing.
IM_TOP and IM_BACK should be of the same size.
```

But what images script combines?

Also in the `poisson_reconstruct.py` what is these `gxs,gys,gxd,gyd`? This is X and Y coordinates of something?

```
[gxs,gys] = get_grads(ims)
[gxd,gyd] = get_grads(imd)
```
Is the alpha blend done here?

```
elif mode=='blend': # from recursive call:
# just do an alpha blend
gx = gxs+gxd
gy = gys+gyd
```
When I replace '`blend`' instead 'max' in function` blit_images()`, nothing happens?