Relative root-dir-path on windows seems to throw NullPointerException

[Jason Finch]

Hi,  I'm not sure if this is the correct place to track issues so I do apologise if so, it's only a minor issue.

On windows, when running:

smartsprites --root-dir-path .\

Generates:

INFO: Writing sprite image of size 16 x 32 for sprite 'common-sprites' to commonsprites.png

Exception in thread "main" java.lang.NullPointerException

        at org.carrot2.labs.smartsprites.resource.FileSystemResourceHandler.getResourceAsOutputStream(FileSystemResourceHandler.java:74)

        at org.carrot2.labs.smartsprites.SpriteImageBuilder.writeSprite(SpriteImageBuilder.java:234)

        at org.carrot2.labs.smartsprites.SpriteImageBuilder.buildSpriteReplacements(SpriteImageBuilder.java:168)

        at org.carrot2.labs.smartsprites.SpriteImageBuilder.buildSpriteImages(SpriteImageBuilder.java:81)

        at org.carrot2.labs.smartsprites.SpriteBuilder.buildSprites(SpriteBuilder.java:206)

        at org.carrot2.labs.smartsprites.SpriteBuilder.buildSprites(SpriteBuilder.java:146)

        at org.carrot2.labs.smartsprites.SmartSprites.main(SmartSprites.java:46)

replacing root-dir-path will a fully qualified path works as expected.