Raindrops exercise

Two purposes for this PR:
* To check that multiple exercises can share the common location for Ceedling vendor files
* To discuss how to handle C-string return values in the xC track

Regarding the latter, I went with the approach of passing in a char buffer as a parameter:
``` C
char* convert(char *buffer, int buffer_length, int drops);
```
My goal for this function prototype was to set aside some memory for the return pointer, which would live past the function exit.  Does anyone have a more elegant approach for the function prototype in mind?