Added documentation for composite().
Fixed indentation on README example.
Added initial code to composite.js.

Also required it in index.js. Basic composition working.
Changed geometry() to accept a custom geometry command.

This allows us to do:

gm("base.png").composite("changes.png").geometry("+100+150")

without having to specify the original image dimensions.
Accept an optional masking image in composite()
Added test for composite()
Modified geometry() as discussed on #332.

Made it only accept a custom geometry command if the number of arguments is strictly one.
Added test for single-argument geometries
Merge pull request #332 from jdiez17/composite

Add support for composite