Fix build errors

These changes were required in order for me to build the project
successfully and have all tests pass.

Apache Maven 3.0.4 (r1232337; 2012-01-17 00:44:56-0800)
Maven home: c:\Projects\apache-maven-3.0.4
Java version: 1.7.0_13, vendor: Oracle Corporation
Java home: C:\Program Files\java\jdk1.7.0_13\jre
Default locale: en_US, platform encoding: Cp1252
OS name: "windows 7", version: "6.1", arch: "amd64", family: "windows"
Fix test issue

The non-default-output-dir test was incorrectly deleting its source
images in addition to its sprites.
Add support for 'sprite-scale' directive

The new sprite-scale directive applies to an entire sprite, and tells
the tool that the intended usage is for high DPI devices with the
specified pixel ratio. Normally, this value is "1"; for devices like the
Retina display, the recommended value is "2". Thus, any image you add to
a sprite with sprite-scale: 2; should have double the dimensions.

The sprite-scale directive takes a float as input. If the scale is not
1, it emits an additional background-size tag for each sprite reference,
and automatically adjusts the background-position based on the scale
ratio.

If the tool detects the scale ratio will result in fractional values
(usually an indication of an error), it will emit a WARNING.
Update tests to support 'sprite-scale' directive

Added a few new tests to validate the sprite-scale directive is working
as expected.