Add a sbet writer

This writer can create sbet files, which store IMU data. Since the sbet
format is pretty dumb (just a struct), this writer is pretty dumb as
well.
Allow faux::Reader to report 0 for getNumPoints

This will be used for simulating readers that have unknown point counts.

This is implemented by a optional boolean argument to one of the faux
reader's constructors. This argument forces the faux reader to report
zero for getNumPoints(), even thought it in fact will provide the number
of points as requested in the second argument.
Merge pull request #324 from gadomski/sbet-writer

Add a sbet writer
Merge pull request #328 from gadomski/feature/faux-reader-unknown-point-count

Allow a FauxReader to have an unknown point count