Provide better version information in CouchbaseEnvironment

Several problems arise when getting version information from the CouchbaseEnvironment:

• packageNameAndVersion() gives the information from core
• userAgent() gives the information from core
• toString() includes the packageNameAndVersion entry, but it is yet again from core
• there is no way of only retrieving the client's version (as described in the build config)
• there is no way of only retrieving the client's build number (as set during build through git describe)

The aim of this change is to fix all the problems above.