fixes documentation of nqp::sha1

Fixes documentation of nqp::sha1 (it's a serialization context opcode, not a
string opcode), and reorganizes its test code.
implements nqp::exit
implements nqp::sleep
Merge remote-tracking branch 'upstream/master'
Merge pull request #6 from thecabinet/master

cleanup of nqp::sha1; implement nqp::exit and nqp::sleep