Incorporate feedback from mailing list about prometheus_client_php

[schnipseljagd]

https://prometheus.io/docs/instrumenting/writing_clientlibs/
According to the documentation I think these are MUSTs we have to do:

• add inc, dec, incBy, decBy to the Gauge
• rename increase to inc on the Counter
• add a default collector registry which holds the it's state on a PHP class.
• validate label names according to https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels

The redis protocol does not support doubles natively.

The current field content or the specified increment are not parsable as a double precision floating point number.

http://redis.io/commands/hincrbyfloat
However AFAIK redis stores the values internally as double so this shouldn't be a problem.
Just take a look into the java world: https://github.com/xetorthio/jedis/blob/master/src/main/java/redis/clients/jedis/Jedis.java#L596

[schnipseljagd]

I changed the naming to be more similar to the existing libraries, according to the style guide.
I updated the readme and added some usage examples.

[bracki]

I consider this done with the changes from #12 and #13.