EMI: Improvements to the sound system

This is a collection of fixes and improvements to the sound system in EMI.

The most notable changes are the following improvements to music playback (see also my blog post about this [here](http://akzgsoc.blogspot.fi/2014/07/for-past-week-ive-been-working-on.html)):
- Cue point data from .jmm files is now respected, so music tracks will start at the correct position and loop the correct sub-section of the track
- Music syncing is implemented (noticeable for example when entering the fountain closeup in Melee town)
- When the music track changes, the music now transitions with a cross-fade

The sounds are now also saved and restored properly.

f391e57 contains a fix for an apparent bug in common MP3 streaming code, which causes `MP3Stream`'s seek method to not behave as expected when seeking backwards. This should perhaps be sent to ScummVM for review as well?
endian.h defines the MKTAG macro which is used for PoolSound::getStaticTag.