Enhance DeleteTree with provisions for retrying on set intervals

[dsidirop]

Retrying comes in handy when having to deal with the quirks that crop up when we try to deploy websites on IIS (running w3p.exe processes lock the directory and prevent < RemoveDir > from working as intended):

https://social.msdn.microsoft.com/Forums/vstudio/en-US/cb6acfa3-88e8-43c9-9a8b-0001b5ecf4e7/random-build-failures-on-removedir-task-error-msb3231-unable-to-remove-directory?forum=tfsbuild

Something like:

< DeleteTree
Directories="..."
Retries="3"
RetryDelayMilliseconds="300" >

Would be nice I guess.

[dsidirop]

Pull request:

#287