Added module win_domainjoin.

win_domainjoin allows Ansible to join a windows computer to a domain. Parameters include the ability to auto-restart after a successfull join (this is turned off by default but should possily be enabled) and which OU path (by DistinguishedName) to put the computer into. 
Parameter name is capitalized here, but not when used below.  I know PowerShell variables aren't case-sensitive, but code and docs should at least match.