This enables pushState support in Express

Since express doesn't handle 404 or any other errors explicitly this 
simply exploits that fact and adds 1 final piece of middleware to the
chain; this has the effect of serving up `generated/index.html` on any
request that would yield a 404

In conjunction with enabling pushState for Backbone/Angular/Embers 
routing abstractions this basically simulates what having a server
configured to work with HTML5 pushState works like. Yay? :)
This solves apiProxy eating everything

But also means people will have to provide a prefix for api endpoints.
I'm torn on this one, I know we used to have a hardcoded prefix in here
but if people are creating a single page app and using HTML5 pushState
all the examples I've found indicate that a prefix is used.
stash conflict
Switches lineman from libsass to ruby sass
0.8.0
Should only include sass in the "common" phase.
Previously, would have included in every single
phase, which is a bit pointless (and slow!)
0.8.1
killed some strangely defensive code.
you can now remove tasks just like prepend/append
documentation
0.8.2
Fixes #81
0.8.3
organizes our options printout in `lineman help`
add a new option to skip example files.

Fixes #82
0.8.4
make travis happy b/c :heart:
More quality of life for the pushState feature

- adds some docs at the top for apiProxy.prefix
- if users have pushState enabled but forget to include the prefix, they
will get a warning from `requiresConfig`
- customized logging output when pushState is enabled and an apiProxy
prefix is included
- factored out the common proxyError handler
- split the apiProxy concerns relating to prefixMatching into its own
function `prefixMatchingApiProxy`
Adds some readme love
Merge pull request #60 from testdouble/push-state

This adds the option to enable `pushState` simulation support in Express