Fix hang when the app writes lots of stdout/stderr

Hi there - spring enabled commands when occasionally hang for me when trying to run them, which turned out to be due to excessive stuff being written to stdout on application startup, which would eventually fill the buffer and block. On my system, it would block after 1024 bytes on stdout.

Possibly this is the same problem being experienced in  https://github.com/rails/spring/issues/265.

This commit adds a thread to read from the PTY during eager load, and echo the result to the spring log file.  WDYT?