Simplify how content gets mangled.

Simplify how content gets mangled.

The many layers of abstraction were unnecessary at this point so I've
pared the code down to the bare minimum based on this POC code:

https://gist.github.com/1343603

Now how we mangle content is based on a wee hash that only takes into
account the current action and the cm (i.e content mangler)
parameter. We decide what to pass into the mangler basde on a wee hash
in ContentMangler which defines the relevant parts of the stash to
pass in. A ContentMangler::Transformer now simply returns a hash that
will be reintegrated into the stash, none of this
action-at-a-distance.

The things that used to work before (diffs & perl code) should
continue to work. I've also added a POC that will PODify blobs
accessible by setting the cm query parameter to doc e.g

http://localhost:3000/Gitalist/master/blob/lib%2FGitalist.pm?cm=doc