package.json: add a repository field

NPM kept complaining about this repository field missing.
package.json: use devDependencies throughout

None of the dependancies listed are actually required for *using*
TinyMCE, but merely when building it. The distinction is mainly an
issue of clarity and correctness, as TinyMCE isn't used as a Node
package proper.
package.json: prevent publishing of TinyMCE to NPM

NPM isn't a node package as such, and thus isn't terribly useful as a
dependancy. As a result, it probably shouldn't be published to NPM,
and this flag prevents it.
Merge pull request #307 from danchr/package-json

Minor tweaks to ‘package.json’