Just some helpful information I found while trying to program atmega328/atmega328p on a breadboard without a crystal.
Bootloader for atmega328 using the 8MHz internal clock (Works in Arduino 1.6.5)
Download and extract this file to your Documents/arduino/hardware folder. The result should be two new folders and a boards.txt under User/Username/My Documents/arduino/hardware/breadboard/avr
This adds a new entry "Atmega328 on a breadboard" to your Tools>board list in the Arduino IDE.