Adds some generic socket option setters
Merge remote-tracking branch 'upstream/master'
Adds socket option setters

Adds the required constant in libnanomsg.
Adds the socket option setters in main lib.
Changes the parameter type of set_survey_deadline to &Duration.
Replace transmute calls by a double cast

In numeric option setters, the transmute call has been replaced by a double cast of the form '&opt_c_local_value as *const _ as *const c_void'.
Fixes issue #42 : Improve drop implementation

Modifies drop function of Socket struct to call nn_close instead of nn_shutdown.
The shutdown feature will be exposed later, see issue #43
Merge pull request #41 from blabaere/master

Adds socket option setters