Use peer connect/express module

The `connect` version used by this middleware should be the same as that used by the application itself. This patch uses an installed `connect` or `express` instance instead of it's own copy. `peerDependencies` can't be used in package.json since these would be optional.

Alternatives are 

1. setting `peerDependencies: { connect: ... }` and publishing a separate `express-session-file` that peer-depends on `express`

2. use dependency injection, eg `FileStore = require('connect-session-file')(connect)` - clumsy when using express since it doesn't expose it's connect dependency: `require('connect-session-file')(require('express/node_modules/connect'))`