Update nomenclature referring to configs, apps, etc

[jbjonesjr]

Mapfish is an application or plugin that resides on a server. This should be referred to as the app.

Mapfish at any one time can have multiple configuration sets outlining options in different environments. These can be called projects.

"Clients" may have access to all or a subset of projects.

[jbjonesjr]

Calling a client parameter request is a "spec" is wrong as well. While this is the specification/contract between the client and the server for how he would like his data to be processed, this typically means a concrete contract, not an ad hoc definition. This would be better named as a "print request."