Refactored bits of gpio_api.c

In between the last two commits, I added debugging lines using serial to
try and locate the line of code that was causing the issues with GPIO.
However, didn't get anywhere with this because the gpio_write function
is defined in a header file, rather than in an implementation file, so
the printf function can't go there. As a result, it's just refactoring.
Updated .gitignore to ignore my uVision Project