replace "$dir$ + $path$" with Path.Combine($dir$, $path$)

this may help resolve issue #1515

Note that I used ReSharper's "Search with Pattern" feature to find these.  Pattern:
`$dir1$ + $str1$`
$dir1$ : identifier matching `[Dd]ir{1}\b`, case sensitive
$str1$ : identifier matching `^[a-zA-z]+(\w*[a-zA-Z0-9]\.[a-zA-Z_])*\w*$` [src](http://social.msdn.microsoft.com/Forums/en-US/vsx/thread/a3fb0ee1-3340-4eaa-b749-aafe8675b102)
Replace pattern: `System.IO.Path.Combine($dir1$, $str1$)`
[x] Shorten references

There were also a few results which added `Settings.PathSeparator` and/or `\` directly, and I tried to avoid fixing those lines, fearing possible breakage.

Untested beyond build and run in debug mode.