[FEATURE] Attribute "maxItems" on Form / MultiUpload

Added due to a need arisen in a legacy site but is useful enough that it should be made public even if FED is obsolete. If and when the Multiupload ViewHelper gets ported, this will then be included.
Merge pull request #24 from NamelessCoder/maxitemsonmultiupload

[FEATURE] Attribute "maxItems" on Form / MultiUpload