WIP: expanding DICOM docs
DOC: adding new DICOM introduction

Make rather large document trying to explain the mysteries of SOP
classes, IODs and other terrors.
DOC: updating the doc from Nolan's comments

Also, some more reading on DIMSE, still somewhat confused.
DOC: Added info about how blocks of private elements are reserved
DOC: Improve description of private block reservation.

Added reference to relevant portion of the standard.
DOC: fix typo
Merge pull request #3 from moloney/dicom-docs

DOC: Added info about how blocks of private elements are reserved
DOC: rewrite the standard / private section

Add quote from the standard and an example.
DOC: small clarification of command set
DOC: remove reference to private data dictionaries

Thinking about it, they don't make much sense because the standard
requires that private tags should be able to move around in the presence
of another manufacturer reserving the space.

Add part of standard pointing out this last point.
Merge pull request #174 from matthew-brett/expanding-dicom-docs

MRG: writing document for DICOM fundamentals

Try to explain DICOM basic terms in a moderately short document.