Create PostgreSQL metrics dynamically for older version compatibility

[bmxpert1]

The current postgresql plugin only works with postgres versions 9.2+ due to lack of some columns in versions older than that. This PR fixes that by dynamically creating metrics based on the columns returned from the pg_stat_database table.

[bmxpert1]

For reference these are the columns returned from pg_stat_database, by postgres version:

pg version      9.2+   9.1   8.3-9.0   8.1-8.2   7.4-8.0
---             ---    ---   -------   -------   -------
datid            x      x       x         x         
datname          x      x       x         x  
numbackends      x      x       x         x         x
xact_commit      x      x       x         x         x
xact_rollback    x      x       x         x         x
blks_read        x      x       x         x         x
blks_hit         x      x       x         x         x
tup_returned     x      x       x
tup_fetched      x      x       x
tup_inserted     x      x       x
tup_updated      x      x       x
tup_deleted      x      x       x
conflicts        x      x
temp_files       x      
temp_bytes       x      
deadlocks        x      
blk_read_time    x      
blk_write_time   x      
stats_reset      x      x

Actually looking at this now I don't think this plugin will be able to fully support anything <= 8.0 because of the lack of datname column. Could work around that but I'm guessing this is more of a non issue.

[sparrc]

@bmxpert1 Great! thank you for the contribution, could you put a link to this PR in the CHANGELOG and also put the above version compatability graph in a README file in the postgresql directory?

thanks!

[bmxpert1]

No problem, I'm loving telegraf.

Thank you!