Implemented SPI interface

.gitmodules

@@ -10,3 +10,6 @@
 [submodule "adc"]
 	path = adc
 	url = git@git.blackmark.me:avr/adc.git
+[submodule "spi"]
+	path = spi
+	url = git@git.blackmark.me:avr/spi.git

README.md

@@ -63,11 +63,10 @@ The PWM mode of the hardware timer can also be used. Additionally, if PWM is nee
     - [ ] Type-safe convenience functions for writing and reading basic data types
     - [ ] Provide an example showcasing the usage and possibilities
 - SPI
-    - [ ] Separation of interface and backend-driver
+    - [x] Separation of interface and backend-driver
-    - [ ] Support all hardware provided configuration options
+    - [x] Support all hardware provided configuration options
-    - [ ] Compile-time check for correct hardware configuration
+    - [x] Compile-time check for correct hardware configuration
-    - [ ] Software backend driver
+    - [x] Provide an example showcasing the usage and possibilities
-    - [ ] Provide an example showcasing the usage and possibilities
 - I2C
     - [x] Separation of interface and backend-driver
     - [x] Support common usage with easy extendability