API to return server side settings #3136

It could be useful, for plugins or remote applications, to be able to read a set of values from Piwik server.

Implementation:

• New API function eg. API.getSettings
• This API would simply return the values from the config file under section [APISettings]

This would be useful for example for a Piwik Mobile SDK. It would return [ batchsize, interval, ttl ]

(In [6319]) Fixes #3136 will be used in the upcoming SDK

(In [6320]) Refs #3136 XML test