Allow users to ignore certain fields in their structs during binding

I have some objects which I would like to read in from Form data, but those objects have unexposed fields which break the reflection. This pull request allows users to mark fields as "ignored", very similar to how Gorp has ignored fields in models.

Example:

```go
type UserModel struct {
	Username      string `form:"user_username"`
	Password      string `form:"user_password"`
	Email         string `form:"user_email"`
	authenticated bool `form:"-"`      // This field will be ignored
}
```