Try out using SimpleGMService instead of a pool of gm processes

Switches from using PooledGMService to SimpleGMService (this idea was suggested [here](http://stackoverflow.com/questions/23846193/multiple-image-opperations-in-a-single-process-with-gm4java/23873353?noredirect=1#comment36868347_23873353)). Performance benefits on an EC2 c3.xlarge instance resizing 77 x 30MB images to 200x200 px thumbnails is below:

## SimpleGMService

| num jvms | num workers | time (sec) |
|----------|-------------|------------|
| 1        | 1           | 93         |
| 1        | 2           | 61         |
| 1        | 3           | 44         |
| 1        | 4           | 40         |
| 1        | 8           | 40         |
| 2        | 2           | 40         |

## PooledGMService

| num jvms | num workers | time (sec) |
|----------|-------------|------------|
| 1        | 1           | 97         |
| 1        | 2           | 62         |
| 1        | 3           | 44         |
| 1        | 4           | 40         |
| 1        | 8           | 40         |
| 2        | 2           | 40         |

## Command Line References

| name                       | time (sec) |
|----------------------------|------------|
| imagemagick                | 120        |
| graphicsmagick (batchfile) | 86         |