Added float conversion functions to ByteVector.

I added following methods to ```ByteVector```. 
Also added some CMake checks relevant to floating point format and byte order. It changes TagLib to require IEEE754-compliant floating point types, but I believe we can assume that every system that may host TabLib is like that.


```
toFloat32LE()
toFloat32BE()
toFloat64LE()
toFloat64BE()
toFloat80LE()
toFloat80BE()

fromFloat32LE()
fromFloat32BE()
fromFloat64LE()
fromFloat64BE()
```
(```fromFloat80*``` may be complicated, so I skipped them for now.)