Get new EetFile main API object

- Get a new object 'EetFile' for usage in the main API.
- _EetFile is an internal representation for an EFL eet file object.
- EetValue is an object wrapper for values obtained using 'read'.
- The EetValue object implements the Drop/Show traits (so values are dealloc'ed automatically).
- EetValue implements a getter for returning a pointer to its internal value (getVal).
- Adapt the test_read_eet.rs to use these changes.