Addition of FIFO Control and Status

[glenntamayo]

This library does not have methods to enable FIFO and select one of its modes.
The setFIFOMode will select one of the four available modes - Bypass, FIFO, Stream and Trigger.
The getFIFOMode will return the byte set in the FIFO_CTL register.
The getFIFOStatus will return the number of data values stored in FIFO.

[SDAMcIntyre]

It doesn't look like sparkfun have looked at this project in a while. :(

[nseidle]

Hi Glenn - Sorry, someone should have contacted you a lot sooner. This looks like a good addition. I have a few changes I need to see.

The use of magic decimal numbers is not ideal. It would be a lot easier for me to read and check if you used bit wise operators such as (and this is pseudo code, I haven't read the datasheet in many years):

writeTo(ADXL345_FIFO_CTL, (1<<FIFO_MODE_STREAM));

Or something of the like.

And passing in a String to any function is (almost always) a bad idea. Using the String constructor will add a lot of overhead any time someone links in the library. I would prefer the user call a function called

myAccelerometer.enableStreamMode();

that in turn called your setFIFOmode with a define. Such as:

ADXL345::enableStreamMode()
{
    setFIFOMode(FIFO_MODE_STREAM);
}

Again, sorry we're so late to the party and thank you for your contribution. If you're interested in continuing this PR, please change it with the above recommendations.