Minor fixes

I've changed the copypasted constant type and moved both of them to the .m file. The thing is static variables will be created every time the header is included (as per http://stackoverflow.com/questions/92546/variable-declarations-in-header-files-static-or-not) and this was probably not its intended use. Moreover, I've improved header's doc a bit, since borderWidth is set to 1.0 and not to 5.0, and I've replaced the disambiguous gray (perhaps could be light gray or dark gray?) with explicit UIColor name.