Fix cp669

Added check for None in assignment on members
Fix cp14452

Removed check for _step being null as the indices method takes care of _step being null.
Fix cp21401

Added a __doc__ that is set to null. When someone prints mmap.__doc__ it will print None
Fix cp19258

Add check if running on 64-bit OS and change directory accordingly.
Fix cp7827 by changing the minimum size
Fix cp7159 by adding the requested methods
Fix cp23205 check for invalid command in system()
Update to use Encoding.GetEncoding
Fix cp28220 - update error message
Merge pull request #185 from ife/master

https://ironruby.codeplex.com/workitem/7406