Compile assets on postinstall in production environments

[rylnd]

We need to run gulp to get things to work on heroku, and we're currently using the fact that it sets NODE_ENV to determine that we're on heroku.

Closes #18.

[rylnd]

@itsthatguy I noticed another discrepancy here: We're referencing both bower and gulp in our package.json template, but do not declare either as a dependency.

I would argue that in order to allow a "compile elsewhere" scenario, gulp should be in devDependencies, but bower is going to be required regardless. Sound right?

[itsthatguy]

@rylnd we merge multiple package.jsons together

https://github.com/itsthatguy/generator-spire/blob/master/app/templates/_package.react.json#L12-L24

[itsthatguy]

That should probably be moved to the default package.json file though