unix: allow disabling of SIGUSR2 signal handler

Allow the user to override the installation of a SIGUSR2 signal handler
by having him set NODE_HEAPDUMP_OPTIONS=nosignal in the environment.

Fixes #21.
doc: add caveats section, document memory usage

Due to fork()'s copy-on-write behavior on modern Unices and the fact
that a major garbage collection cycle is performed right afterwards,
creating a snapshot may require about 2x the amount of memory that
was in use right before the snapshot.