v2.7.5: Merge pull request #218 from klauer/enh_guid

Fixes

• Relaxed end-of-pragma-line handling (any combination of ; and newline are
  all accepted).
• Reworked XTI file loading for "devices" and "boxes". This aims to be more
  compatible with TwinCAT, which does not always relocate XTI files to be in
  the correct hierarchical directory location. It pre-loads all XTI files, and
  when the project is fully loaded, it dereferences XTI files based on a key
  including class, filename, and a small PLC-unique identifier.
• Better handling of data types in the project parser. Now supports data type
  GUIDs, when available, for data type disambiguation. Note that these are not
  always present.
• Better handling of references, pointers, and pointer depth.

Development

• pytmc db --debug allows developers to more easily target exceptions
  raised when generating database files.
• Added more memory layout information for the benefit of other utilities such
  as ads-async. Its ADS server implementation in conjunction with pytmc may
  be a good source of information regarding PLC memory layout in the future.
• Started adding some annotations for clarity. May retroactively add more as
  time permits.