Implemented a new ini plugin

Because the simpleini plugin did not work too well for mounting the specification file and even failed to parse other ini files (e.g. samba), I wrote a new one. It uses a slightly patched version of the very small inih library (directly embedded). Although it works fine for now, it is still work in progress though. 

Improvements compared to simpleini:
* graceful handling of sections (keys are subkeys of their sections)
* support for comments (comments, are appended to the next non-comment key)
is this a duplicate? we should consider to have a more handy ELEKTRA_SET_ERROR