self.meta -> self._meta
a minor test bugfix (will not affect travis)
store bunit in metadata
added unit conversion to a lot of functions
unit(1) instead of 1
whoa u.one is fast
u.one -> u.dimensionless_unscaled
strip data units if data come in with units (let spectral_cube handle
them).  Relative import in tests.  Use check_endian'd data if it's
checked.