KeyWords_PS: add invoke-webrequest

http://technet.microsoft.com/en-us/library/hh849901
Unfortunately I cannot find documentation for it, however it is true for PowerShell 3.0 and later. I found 2 other aliases as well, I can add them if you are interested in merging this

~~~
PS > Get-Alias -Definition Invoke-WebRequest

CommandType     Name
-----------     ----
Alias           curl -> Invoke-WebRequest
Alias           iwr -> Invoke-WebRequest
Alias           wget -> Invoke-WebRequest
~~~