Use std::chrono::nanoseconds for Timer construction

[barbu110]

The constructor of Timer now takes std::chrono::nanoseconds as parameter.

This choice instead of the more general std::chrono::duration<Rep, Period> comes from the fac the nanosecond is the smallest unit supported by the timespec structure.

A timer can now be constructed as follows:

/* using duration specializations */
set_timeout(std::chrono::seconds{2}, /* ... */);

/* or using literals */
using namespace std::literals::chrono_literals;
set_interval(2s, /* ... */);

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This pull request fixes #16.