latest/nightly version, may be unstable

Merge branch 'master' of github.com:stnava/ANTs

...

ENH: Add clarification.

ANTs Version 2.1.0

PTBP

with ITK v4.6.0 and VTK 6.1

with ITK v4.6.0 and VTK 6.1

Based on ITK v4.5.0; Doc at http://stnava.github.io/ANTsDoc/: bug fix…

…es from last rc ... linux

Based on ITK v4.5.0; Doc at http://stnava.github.io/ANTsDoc/: bug fix…

…es from last rc

Based on ITK v4.5.0; Doc at http://stnava.github.io/ANTsDoc/