Adding File, Line, Stacktrace

This is just an initial approach to get the ball rolling in reference to #63 that I implemented last night really quickly. It needs test, hardening,etc. I am all for a much more permanent solution, but this is at least a start. The two things that need to be addressed are whether or not we are losing context by combining the file & line number like so:

```bash
INFO[0006] Request completed                             file=/bursa/src/bursa.io/kernel/kernel.go:96
```

and how to format the stack trace. Right now it is pretty gross:

```bash
DEBU[0001]                                               file=/bursa/.godeps/src/github.com/Sirupsen/logrus/logger.go line=100 stack=goroutine 3 [running]:
bursa.io/middleware/logtext.getTrace(0x756ec0, 0xc2100c8240)
	/bursa/src/bursa.io/middleware/logtext/logtext.go:46 +0x6e
bursa.io/middleware/logtext.(*Logtext).Format(0xc2100915e0, 0xc2100928c0, 0x43029a, 0x7d9860, 0xc21001ea80, ...)
	/bursa/src/bursa.io/middleware/logtext/logtext.go:36 +0x181
github.com/Sirupsen/logrus.(*Entry).Reader(0xc2100928c0, 0x376d4605, 0xc2100928c0, 0x0)
	/bursa/.godeps/src/github.com/Sirupsen/logrus/entry.go:41 +0x38
github.com/Sirupsen/logrus.(*Entry).log(0xc2100928c0, 0x5, 0x0, 0x0)
	/bursa/.godeps/src/github.com/Sirupsen/logrus/entry.go:84 +0x1e9
github.com/Sirupsen/logrus.(*Entry).Debug(0xc2100928c0, 0x7f64f0934a80, 0x1, 0x1)
	/bursa/.godeps/src/github.com/Sirupsen/logrus/entry.go:109 +0x75
github.com/Sirupsen/logrus.(*Logger).Debug(0xc21004a180, 0x7f64f0934a80, 0x1, 0x1)
	/bursa/.godeps/src/github.com/Sirupsen/logrus/logger.go:100 +0xa3
github.com/Sirupsen/logrus.Debug(0x7f64f0934a80, 0x1, 0x1)
	/bursa/.godeps/src/github.com/Sirupsen/logrus/exported.go:61 +0x48
bursa.io/controller/home.HandleSignup(0x7f64f0ab9ce0, 0xc210092740, 0xc21005e750)
	/bursa/src/bursa.io/controller/home/home.go:36 +0x2f7
net/http.HandlerFunc.ServeHTTP(0x902f78, 0x7f64f0ab9ce0, 0xc210092740, 0xc21005e750)
	/usr/local/go/src/pkg/net/http/server.go:1220 +0x40
github.com/gorilla/mux.(*Router).ServeHTTP(0xc21005a4b0, 0x7f64f0ab9ce0, 0xc210092740, 0xc21005e750)
	/bursa/.godeps/src/github.com/gorilla/mux/mux.go:98 +0x21c
github.com/codegangsta/negroni.func·001(0x7f64f0ab9ce0, 0xc210092740, 0xc21005e750, 0xc210091c40)
	/bursa/.godeps/src/github.com/codegangsta/negroni/negroni.go:41 +0x4c
github.com/codegangsta/negroni.HandlerFunc.ServeHTTP(0xc210093e60, 0x7f64f0ab9ce0, 0xc210092740, 0xc21005e750, 0xc210091c40)
	/bursa/.godeps/src/github.com/codegangsta/negroni/negroni.go:24 +0x4a
github.com/codegangsta/negroni.middleware.ServeHTTP(0x7f64f0ab8808, 0xc210093e60, 0xc210091a80, 0x7f64f0ab9ce0, 0xc210092740, ...)
	/bursa/.godeps/src/github.com/codegangsta/negroni/negroni.go:33 +0x82
```

In this example you add this functionality by:
```go
log.SetFormatter(logtext.NewLogtext(new(log.TextFormatter), true))
```
This should be a field on `entry` just like `Time`, `Level` and `Message`. It should be up to the formatter whether to show this. I don't want to enable this by default.