Partial fix for CP21478.
https://ironruby.codeplex.com/workitem/7407

Added missing methods
IOWrapper: pos, eof, eof?
GZipReader:  pos, tell, eof, eof?
GZipWriter: pos, tell
Merge pull request #216 from ife/master

https://ironruby.codeplex.com/workitem/7407