Moving iPhoto to external drive on a MacBook Pro (Retina, 13-inch, Mid 2014) with 8 GB 1600 MHz DDR3
iPhoto was built with this sort of flexibility in mind: it is possible and in fact iPhoto makes it quite easy. 


Find your iPhoto library. You'll most likely find it in your Pictures folder. If it's not there, open iPhoto while holding the option key, and you should get a window similar to the one below. The path to your library is displayed under the library selection box.





Move your library to the external hard drive. 
Open iPhoto with the option key pressed to open the library selection window I talked about in Step 1. Hit the Other Library... button, and find the library on your external hard drive. Pretty easy!


Protips: 


The last library you opened becomes your "default" library. That's the one iPhoto opens automatically when you don't press option. If that library isn't available, say your external HDD is not connected, the library selection window will open automatically and list the libraries it can find. 
I keep a library in my computer, but every year or so, when it gets
too full, I move it to an external hard disk. You can have as many libraries as you want! Pretty useful if you don't feel like having a single massive package.
iPhoto supports Apperture libraries, as you can see from my screenshot. d: 


Let me know if you have any further questions!