Fixed a black hole state problem in AEBlockScheduler

Many thanks to AndreasHerbig for diagnosis and solution.
Fixed a podspec issue

Closes #64
Updated TPCircularBuffer

- Don't modify mDataByteSize buffer values on return from TPCircularBufferDequeueBufferListFrames
- Addressed a few warnings when building for 64-bit
Use float version of log10 in level meter calculation to avoid overflow issue

Closes #65
Thanks to rlopezdiez
Further tweaks to level metering
Make sure all AudioUnits continue rendering during screen lock.
Fixed an AEMixerBuffer issue with multiple dequeues in one time slice; increased max sources
Fixed bug calculating NSTimeInterval to timeStamps that are in the past

Time difference is calculated from two uint64_t (unsigned) values. To get correct results when the first value is less than the second value, we have to cast the result to an int64_t (signed) value.
AEFloatConverter: Make sourceFormat settable
Merged in patch from http://forum.theamazingaudioengine.com/discussion/104/aeaudiounitfilter-couldn039t-setup-converter-audio-unit-error#Item_7.
Added mixer channel count check to prevent -10877 errors.