Git ant task

Hey Rich,

Here's the Git Compare ant targets that can be used to only zip up files and images that have been modified. Here are the steps to use:

1. Run `ant compare-with-older-branch -Dolder.branch=OLDER_BRANCH` in appropriate dir (../learning-paths, ../tutorials, ../using-liferay-portal, etc)

    This creates a `git-modified-list.txt` file in the dir, which lists all the additions, modifications, and deletions between the older branch you specified, and the current branch you're on.

2. Run `ant dist-diffs`

This zips up the new markdown and images that have been added/modified to a `diffs.zip` file. The ant task that was used in step 1 must be called first, because this target reads and zips the files that are listed in the `git-modified-list.txt` file.

Thanks!

@jhinkey