Ability to start in a separate folder with presentation.md...

$Root is the current location. Files are searched in $Root at first, then in
$Here, the location of httpd.ps1

$ErrorActionPreference = 'Stop'
- to fail on the second run (.Start()) when the server is already started
- for simple Write-Error
Improved start-presenter-for-windows.bat...
- `-ExecutionPolicy Bypass` - the most effective way
- `-File "%~dp0httpd.ps1"` instead of `-Command` - to support all paths
Tabs to spaces, to match the original style.
Minor tweaks in httpd.ps1
Merge pull request #10 from nightroman/master

Improvements for starting on Windows