Support binding a string array to a cmdlet parameter.

Example statement:
  join-path -Path c:, d:, e: -ChildPath temp
Implement combining paths for join-path.

Simple implementation to support directory and file paths. Join-Path should be calling
NavigationCmdletProvider.MakePath(string parent, string child) but it is not
doing this currently:

http://msdn.microsoft.com/en-us/library/windows/desktop/ee126197.aspx
TEST: Fix join-path tests on Unix.
Merge pull request #154 from mrward/join-path

Implemented combining paths for join-path