Store http.Request reference in endpoints.Context (see #2)

This change would just keep a reference to http.Request and use that instead of interfering with internal workings of `appengine.Context.Request()`.

So, instead of this:
```go
req, ok := c.Request().(*http.Request)
if !ok {
  // always fails on production
}
```
we can now just do
```go
req := c.HttpRequest()
// go ahead and use req, no even need for a type assertion
```

(a possible solution for https://github.com/crhym3/go-endpoints/issues/2#issuecomment-18984862)