ENH: Add dtype parameter to linspace and logspace functions.

Many NumPy functions such as arange allow users to define the dtype of the
returned type with a dtype parameter.  This adds this same functionality
to the logspace and linspace functions.
Merge pull request #3482 from jjhelmus/linspace_enh

ENH: Add dtype parameter to linspace and logspace functions.
MAINT: update release scripts for building OS X binaries.

Works with Python 3.x now that bdist_mpkg is ported (thanks @matthew-brett).