added support for multiple servers
added support for multiple servers

PS C:\github\dbatools> $servers = Get-SqlRegisteredServerName sqlserver
PS C:\github\dbatools> Install-SqlWhoIsActive -SqlServer $servers
-Database master
Finished installing/updating sp_WhoIsActive in master on sql2005
Finished installing/updating sp_WhoIsActive in master on
sql2005express\sqlexpress
Finished installing/updating sp_WhoIsActive in master on sql2008
Finished installing/updating sp_WhoIsActive in master on sql2008\sql2k8
Finished installing/updating sp_WhoIsActive in master on sql2012

sqlpadawan/dbatools,dsolodow/dbatools,kozloski/dbatools,wsmelton/dbatools,alevyinroc/dbatools,SirCaptainMitch/dbatools,Splaxi/dbatools