Add support for basic type pointer fields

Update README

Allows models like:

```
type User struct {
	Id    int64
	Name  string
	Email *string `orm:"null"`
}
```

This helps a lot when doing JSON marshalling/unmarshalling.

Pointer fields should always be declared with the NULL orm tag for sanity, this
probably requires documentation.

Add new function SetBasicAuth, and update README

format comments

Update httplib support read data from response buffer, add some testcase...

rename SetAgent and ToXML, and update some testcase

Remove some unnecessary code

Another change to match with logs/log.go (and RFC 5424 specs)

To match with logs/log.go (and RFC 5424 specs)

Makes more sense to Use Debug instead of Info?