Replaced AdafruitMCP4725 directory with git submodule

For easier version control.
Also update Adafruit MCP4725 library to the latest commit to date.

*Submodule / version control informations: [How to nest git repositories; fetch and merge - Stack Overflow](http://stackoverflow.com/questions/10325586/how-to-nest-git-repositories-fetch-and-merge/10325603#10325603)*