Significant changes

Fix a bug where FlexASIO waits until the worst possible time to provide the next output buffer, increasing output latency by one buffer length. See #40.
Added support for ASIOOutputReady(), enabling compatible ASIO Host Applications to achieve a one buffer length reduction in output latency. See #4.
Speed up the beginning of streaming by providing initial buffers to PortAudio (instead of silence). See #40.
Fix a bug where FlexASIO underestimates output latency by one buffer length. See #40.
Fix a bug where FlexASIO provides slightly wrong ASIO timestamp information to the Host Application. See #40.
Change the directProcess ASIO flag from false to true in buffer switch callbacks. This can potentially improve performance with some ASIO Host Applications. See #40.
Various performance improvements that provide more time budget for processing events, making audio glitches (discontinuities) from missed deadlines less likely, especially with small buffer sizes.

Minor changes

Many improvements to the FlexASIOTest test program:
- Add the --input-channels and --output-channels options.
- Preload the input file before streaming, and write the output file after streaming, to avoid performance issues caused by sound file I/O.
- Add a --log-mode option to customize the way the log is output.
- FlexASIOTest.exe has been refactored into a small wrapper around a DLL containing the testing logic, ASIOTest.dll.
- Fill the first buffer before calling ASIOStart() instead of waiting until the first buffer switch.
- Optionally call ASIOOutputReady() when an output buffer is filled.
- Add --buffer-switch-delay-ms option for adding an artificial delay to buffer switch callbacks.
- Exit cleanly when receiving a CTRL+C signal during streaming.
- Call ASIOStop() even when streaming failed.
- Do not wait for a buffer switch if --buffer-switch-count is zero.
Ship PDB files in the installer (again) to allow troubleshooting tools (Process Explorer, Process Monitor, Windows Performance Analyzer, WinDbg, etc.) to understand FlexASIO stacks.