MAINT: update pavement.py vs windows paths + py34
TST: signal: disable a known failure in slepian test

(cherry picked from commit 1e93831d1c92a8e2a844f52f6044050ff542b8d6)
DOC: update 0.15.0 release notes
REL: set version to 0.15.0rc1
REL: set version to 0.15.0rc2-dev