Change username to usertype

Prior to this commit we called this username

however this appears to be more a user type:

```ruby
    # Logs a user into a token.
    # @param [Integer, Symbol] user_type  is the user type CKU_*;
    # @param [String] pin  is the user's PIN.
    # @return [PKCS11::Session]
    #
    # When the user type is either CKU_SO or CKU_USER, if the call succeeds, each of the
    # application's sessions will enter either the "R/W SO Functions" state, the "R/W User
    # Functions" state, or the "R/O User Functions" state. If the user type is
    # CKU_CONTEXT_SPECIFIC , the behavior of C_Login depends on the context in which
    # it is called. Improper use of this user type will raise
    # CKR_OPERATION_NOT_INITIALIZED.
    def C_Login(user_type, pin)
      @pk.C_Login(@sess, string_to_handle('CKU_', user_type), pin)
      self
    end
    alias login C_Login
```
[here](https://github.com/larskanis/pkcs11/blob/master/lib/pkcs11/session.rb)