Postgresql 9.4 and 9.5 compatibility

Worked on the compatibility with PG 9.4
Having full compatibility across different version will be hard mostly
because of using shared memory segment which differ between postgres
major versions.

Will now focus on getting a custom background worker to store
historical data.

Cleanup and optimizations

Add some parameters in postgresql.conf to control what and when metrics are cpatured

Reduce memory footprint for the system metrics

First official release

Testing successfully on PG 9.5.

For now statistics are collected at each Executor hook (Start, Run, Finish, End) in order to get the sql_query appearing immediately; this has a significant impact on performance.

In further release I'll study the possibility to change this behaviour or see if the code can be optimized.

As it is my first C extension version number remain crappy (as it's a fork of pg_stat_statements)