Improving RAM footprint decreasing the memory usage by serial command identifiers

[ddanielvaz]

When using method addCommand, the identifier has only 2 bytes but they are stored in a struct of 35 bytes, this pull request is a proposal to fix it. The default library setup, has 16 identifiers of 35 bytes, using 560 bytes. After improvement it will use 32 bytes, a reduction of 528 bytes, or 94%. It is very significant when using arduino nano that has 2048 Bytes of RAM, an improvement of ~25%.

[cparrapa]

Looks good @ddanielvaz simple change for a big improvement thanks!