PowerShell script version of tcx-strip-heart-rate

PowerShell script version of tcx-strip-heart-rate for Windows users:
- uses the same XSL file and syntax as the original tcx-strip-heart-rate
- has the advantage that it doesn't require Saxon to be installed to do the XSL Transformation; this is supported through the .Net framework used by PowerShell
Merge pull request #1 from pver/patch-1

PowerShell script version of tcx-strip-heart-rate