Playa crashes if a directory contains too many files.

[gavinlaking]

Playa sequentially opens each file in the directory to extract the ID3 tag information for the track collection database. With large directories this causes Playa to fail with Errno::EMFILE Too many open files @ rb_sysopen.

Need to look into:

• Not opening so many files, or closing them before we run out of filehandles.
• Caching the list of files in a directory.
• Caching their ID3 tags in a SQLite database maybe?