Merge pull request #106 from hintjens/master

Started zsys class for various non-portable system functions
Make zctx_destroy restore original sighandler
Merge pull request #107 from felipecruz/feature/context_signal_test

Make zctx_destroy restore original sighandler
Fixed signal handling
- non-portable signal code moved to zsys
- zsys_handlers_set (NULL) to prevent CZMQ installing its handlers
- note: no longer a side-effect of setting zctx_interrupted to 1
- zsys_handlers_reset () at exit to restore default handlers