#49 Consolidate API

[David von Oheimb]

Export only functions that are really needed for client/app use (also reduces documetation need).
Possibly merge source files to allow more "static" (private) functions.

[Martin Peylo]

The following needs to be fixed then:

CMP_PKIMESSAGE_free() is not in exported API while CMP_certreq_new() is exported.