Improvements to logging.

  * The old logging callback has been deprecated and will be removed in
    version 0.11.
  * MA_LOG_LEVEL_DEBUG has been added and MA_LOG_LEVEL_VERBOSE
    deprecated.
  * The MA_LOG_LEVEL option has been deprecated. All log levels are now
    posted to the logging callbacks, except for MA_LOG_LEVEL_DEBUG
    which is only posted if MA_DEBUG_OUTPUT is enabled.

The new logging system works by creating a `ma_log` object. You then
register callbacks that will be fired when a log message is posted. You
can register up to 4 callbacks. You the specify a pointer to this log
object in the context config. This replaces the `logCallback` variable.

The old logging system was specific to context's and device's, however
with the introduction of new APIs this is no longer appropriate. The
new logging system is completely generic with a simple user-data
pointer being used for application-specific data.

This commit adds some helper APIs for retrieving a pointer to the
context's log object:

  * ma_context_get_log()
  * ma_device_get_log()
  * ma_device_get_context()

The MA_DEBUG_OUTPUT option has been improved for Android builds. With
the new system, __android_log_print() will be used instead of printf().