Add .pd_lua extension for Lua

.pd_lua is the required extension for any Lua files written to directly communicate with Puredata, via the pdlua library.
Add examples of .pd_lua files

Added examples of .pd_lua files, which create Lua objects that are
interpreted by PureData.
Merge pull request #232 from strangewarp/patch-1

Add .pd_lua extension for Lua