Examples: Rename "Program" to "MyProgram"

"Program" is very generic and sounds like it might refer to generic
infrastructure. Calling it "MyProgram" makes it clearer, that this
generates a specific type whose name the user might change.
Add comments to examples to explain shader_param

Since documentation is generally lacking right now, the examples are the
best learning material for gfx-rs (at least they are for me). I think
it's important to make them as clear as possible.
Merge pull request #261 from hannobraun/shader-param-doc

Make shader_param use in examples clearer