Adding Stringer implementation for Stacktrace & StacktraceFrame

[aheuermann]

Added String() methods to Stacktrace and StacktraceFrame. Useful if you want to log the same stacktrace to something like syslog.

Here is an example:

github.com/getsentry/raven-go/stacktrace_test.go:85
    trace:  return NewStacktrace(0, 2, []string{thisPackage})
github.com/getsentry/raven-go/stacktrace_test.go:150
    TestStacktraceString:   st := trace()
testing/testing.go:456
    tRunner:    test.F(t)