Removed nillable attribute from elements and cleaned up some whitespace issues. Closes #173
Fixed missing quote in Disk Object schema
Merge pull request #176 from CybOXProject/issue_173

Merging issue_173 branch into version_2.1 branch
Updated Memory Object. Added Block_Type, Memory_Source, Region_End_Address, and is_volatile fields along with a BlockTypeEnum enumeration type. Closes #118
Merged updates from  version_2.1 branch