Add support for Long Paths on Windows (Fixes #125)

This MR adds support for removing directories containing long paths (>260 characters) when using the PowerShell shell implementation. This support is implemented through detection of NTFSSecurity's Remove-Item2 command which has support for long paths, falling back on Remove-Item when it cannot be found.

In addition to this functionality, a workaround involving git fetch and git config --system core.longpaths true has been documented in the FAQ.

For a full discussion of possible solutions and the original problem, please look at #125 (closed)