Is the 3.5 mm headphone jack of a Macbook Pro Retina also a line in port?
Yes, you can use the port as a line in. As detailed in Apple's support article,


Plug the input audio device into the combination audio port using
the proper adapter cable. 
From the Apple () menu, choose System
Preferences. 
Choose Sound from the View menu. 
Click the Input tab.
Change the "Use audio for:" menu selection from Sound Output to the
Sound Input. 
The device type should change from "Internal
Microphone/Built-in Input" to "Line In/Built-in Input."