server: fix for CPU detection

[missinglink]

PR #64 introduced a bug where failing to specify the CPUS environment variable started the server in single-threaded mode:

$ node server/http.js 
cpus NaN

This PR fixes the bug so the default behaviour is now to start the server using all available CPUs:

$ node server/http.js 
cpus 8

CPUS=-1 node server/http.js 
cpus 1

CPUS=0 node server/http.js 
cpus 8

CPUS=2 node server/http.js 
cpus 2

CPUS=999 node server/http.js 
cpus 8

CPUS= node server/http.js 
cpus 8

CPUS=test node server/http.js 
cpus 8

[orangejulius]

Can I get a machine with Infinity cores too please?