Issue 137

Resolves #137 

This pull request adds the following:

**cybox_common.xsd**
* `EndiannessTypeEnum`: Enumeration of endianness types (big, little, middle)
* `EndiannessType`: Implementation of the `EndiannessTypeEnum`
* `Byte_Order` element to `DataSegmentType`
* `Byte_Order` element to `ByteRunType`

**ArtifactObject.xsd**
* `byte_order` attribute to the `RawArtifactType`