[named] provides an identifier for each instrumented client

[esiqveland]

This is a suggestion for providing easily identifiable metrics for individual clients.

Lets you provide an identifier for each instrumented client.
This is very useful when looking at metrics in applications that consume
services from many other services and lets you see which service a client is using.

Adresses #22.

[esiqveland]

Sorry, I wanted to keep this addition in one commit, but all your comments disappeared after I resubmitted :(

Just wanted to say I am open for more discussion as well, I fixed a lot of just style comments and such.

Edit:
Found the comments here: esiqveland@fe45e75#comments

[raskasa]

Ping!

[esiqveland]

Hey, I did not have time to look this over until now.

Fixed the comments and finally the whitespace xD

I like keeping work in one coherent commit, but it does not play very nice with PR comments.
Please tell if you prefer lots of small commits.

Pong!

[raskasa]

I prefer one coherent commit as well. Thanks!

[raskasa]

I've thought a little more. I don't think we should have a name by default.

The library authors designed the library with the intention for users to have one OkHttpClient per application. My assumption is that this happens in a majority of cases. I'm thinking that the name should be null by default.

Then the metric names can look like this by default:

...
com.squareup.okhttp.OkHttpClient.cache-write-abort-count
com.squareup.okhttp.OkHttpClient.connection-pool-count
...

And like this if it has a custom name:

...
com.squareup.okhttp.OkHttpClient.custom-name.cache-write-abort-count
com.squareup.okhttp.OkHttpClient.custom-name.connection-pool-count
...

I can make this change myself in a separate PR. Either way, thoughts?

[raskasa]

Closes #22