Replicate fit decode example

This pull request replicates the decode example from the FIT SDK.

## TLDR

* Loads the provided fit.jar library file.
* Sets up minitest as the test framework.
* Replicates the interface of the existing FIT and TCX gems to read an IO stream and parse the FIT file.
* Tests that we can parse one of my Forerunner 620 FIT files.

## One more time with details:

This means we can now read out the following information from the `file_id` message:

* type
* manufacturer
* product
* serial_number

There are no smarts around what the values mean, but we are checking for invalid values before assigning them to the `file_id` object. The numbers that we get back are the same ones we get back if we run the `FitDecodeFit.jar` example program from the command line:

```
☁  forerunner [replicate-fit-decode-example] java -jar FitDecodeExample.jar 620.FIT
FIT Decode Example Application
File ID:
   Type: 4
   Manufacturer: 1
   Product: 1623
   Serial Number: 3877468399
Decoded FIT file 620.FIT.
```