Added check that all blocks are connected before generating Lua code.
Minor changes to warning dialog.
Disallows "_" as a variable name and checks for warnings before code generation.
Note that lua_compressed.js has not been updated (because of other changes
in progress).
Added backup files to .gitignore.
Added two-variable setter for use with functions returning multiple values.
Updated lua_compressed.js with this and previous changes.
Changed displayed title of turtle_transfer_to block from move to transfer to decrease difference from generated code for user debugging purposes.  (I usually have to debug my programs in Lua.)  Also added help URL.