Add static reverse proxy support for Keter.

No longer have to set up keter bundles for mighttpd or other proxies.
Add example in README.md for Keter reverse proxy.

This is related to previous commit.
Add cabal sandbox files to git ignore
Built-in reverse proxy now supports rewriting headers

This is necessary for using Keter as a front-end to other web
services that may require hacks like Jenkins does, where the
"Location" header needs to be rewritten to use https on every
response.

Example:

```
root: ..
port: 80
ssl:
  # host:
  port:
  key:
  certificate:
reverse-proxy:
  - reversed-host: jenkins-internal.corp.example.com
    reversed-port: 8080
    reversing-host: jenkins.example.com
    ssl: False
    rewrite-response:
      - header: Location
        from: ^http://jenkins.example.com
        to: https://jenkins.example.com
```
Add examples of reverse proxy and header rewriting to configs.
Clean up a comment from an earlier version of the code.
Experimental support for legacy / malformed responses from proxied server.

This is entirely to support behavior of broken web servers that might be
proxied.

The culprit, in this case, is a Jenkins server on Ubuntu 12.04 (installed using
their repositories and running in the default Servlet container). This server
for whatever reason generates responses that have this rare ill-formed set of
headers:

  HTTP/1.1 200 OK
  Connection: Close

With *no content-length* and *no transfer-encoding*.

So this is an initial attempt at working around this.
Demonstrates issue with consuming request from http-conduit.

This attempts to consume the request from http-conduit by using $$+- and
printing a string before and after. The "after" never happens under the
following response header conditions:

  1. HTTP/1.1
  2. Connection: Close
  3. *no* Content-Length
  4. *no* Transfer-Encoding
Demonstrate issue with consuming last "chunk" from http-conduit.

This puts the nail in the coffin, altering the integer passed to isolate will
varyingly cause the response to error out and never reach the second "print"
or cause Keter to respond with a truncated message.
Debugging - print chunk sizes of HTTP bodies through proxy
Fixes for working with legacy servers that poorly handle chunked transfers.

Essentially, some servers when receiving a chunked transfer are observed to
behave poorly on the response side when receiving a chunked encoded request
body from the client.
Merge branch 'master' of https://github.com/AaronFriel/keter
wai version bump
Version bump