plasticine
Prevent overwriting the manifest in the first phase of the build
Since we store the optimization manifest in the build directory, and since it doesn't have a counterpart in the source directory the ManifestResource needs to behave a little differently than your average resource. Instead of reading a file from source and writing it to build we want to read from build if the file already exists, so that we preserve the contents of that file through the first stage of the build.
In one of my projects this PR takes a 22 minute build down to 2 minutes.
Fixes #60 Fixes #41, Closes #58 (Specifying the manifest path shouldn't be needed if we don't overwrite it, and storing it outside the build directory doesn't exactly make sense since it is an artifact of a particular build, or sequence of builds. To say it another way, if you used an existing manifest with an empty build directory you'd end up skipping optimization of images that would need to be optimized.)
