Here's the bill of materials (BoM) of all the parts you'll need to complete this project. You can source the parts through Adafruit or, if you need to, through various online wholesalers like DigiKey. Obviously, I recommend you buy your bits through Adafruit like I do.

1. Magnetic contact switch (Adafruit Product ID 375). You will need one of these for each door and window you want to protect.
2. Adafruit Feather HUZZAH ESP8266 (Adafruit Product ID 2821) or Adafruit HUZZAH ESP8266 breakout (Adafruit Product ID 2471). You will need one of these for each door and window you want to protect.
3. Adafruit Lithium Ion Polymer (LiPo) battery, 3.7v in whatever size you like. I use the 2500 mAh version (Adafruit Product ID 328). You will need one of these for each door and window you want to protect.
4. Five-volt switching power supply with MicroUSB connector (Adafruit Product ID 1995) to power the Feathers. You will need one of these for each door and window you want to protect, as well as one for the Raspberry Pi 3 "control center."
5. Resistors. The voltage divider will use a 1M Ohm and a 470K Ohm resistor. You will need a voltage divider for each door and window you want to protect.
6. Raspberry Pi 3 (Adafruit Product ID 3055) to run the MQTT broker and the Python security system program.
7. A class-10 SDHC micro card for the Raspberry Pi operating system (OS).
8. An account on Adafruit.IO
9. An account on If This Then That (IFTTT).
10. A mobile phone that can receive SMS text messages.

Finally, I recommend a high capacity deep cycle UPS system so your cable/fiber modem, wireless access point, and Raspberry Pi will keep running even when mains power is not available.

Next: Building the Hardware!