Bump dependencies
Add method for series level DICOM insertion

This method enables any image level instance to easily create a DICOM
object by using this method to fill in series/study/frame/patient level
attributes.
Add a set of new DICOM attribute constants.
Add date and time convenience methods

These allows the easy creation of date and time attributes in the proper
DICOM format.
Fixed a structure set reference bug

This fixes an issue where a structure set and its ROIs would not be
properly referenced to their corresponding image series if another study
reference item appeared before the image series' actual study reference
item.

With this commit the code has been made more flexible: It will traverse
the study items until it finds a valid reference (and then it looks no
more).
Apply duck-typing to Coordinate

Before this class required float coordinate values (e.g. raised error if
given an integer), now it will use #to_f on given arguments.
Method for converting image series to voxel space
Adds support for DRR generation

This commits adds initial support for computing digitally reconstructed
radiographs (DRRs) in rtkit, by implementing a radiological path
algorithm in ruby (ray.rb), which uses information from the RTPLAN and
CT Image series to create a projection image, which can be converted to
an RTIMAGE DICOM file.

Note that this is just initial, 'alpha level' code, which definitely has
some issues with peformance, functionality and test coverage. Hoewever,
with some polish, this could become quite usable and enable a relatively
easy way of creating custom DRRs based on given RTPLAN/RTSTRUCT/CT
series.

Note also that RTPLAN or RTSTRUCT ROI image delineation/burn-in is not
supported yet. This will have to be added in a future update.