Convert core documentation to markdown

Converts documentation of core source files to use markdown formatting,
which is cleaner and easier to read than RDoc's default format.

When creating documentation, use:

rdoc --markup=markdown
Performance enhancements

This commit:
* Updates the README to include an example for correct documentation
generation
* Converts the interactive Terminal into an Executable (from Runnable),
giving better control over the runtime.
* Updates the Renderer and Mixer with a higher performance function to
append Transforms.
* Adds the new #+() (alias: #add()) method to Transform, returning a new
component which combines the non-boundary properties of the base
Transform and another (use #add!() to perform this internally).
Performance enhancements II, cleanup

Cleans up code and improves performance by elevating all Executables to
java.lang.Runnable status.

Also finally properly fixes up the issue in the terminal.
Performance enhancements III

This commit includes:

* Rename "Executable" to "Process".
* Better documentation for Process.
* Modify existing Process implementations to better leverage
#background() via automated suspension checking to remove looping within
#runtime().
* Move all event-handling classes and modules to the new "events.rb"
source file to clean up "entity.rb".
* Better Event documentation.
* EventListener components may now be created using only a mask and
block to define its action logic.
* Minor readability improvements.
Graphical component update

Graphical components no longer extend Transform and instead retain their
own internal translation, rotation, and bounding data.  Bounds may
either be a 2D vector (z elements are dropped) or a Polygon described by
an array of X/Y elements of length 4 or greater which must be divisible
by 2.

This commit fixes a bug in the events library and allows a Runnable to
be spawned from a method by passing it as the `proc` argument, thus
emulating Object#send().
Merge pull request #13 from florf/development

Confirmed.