Support minified socket.io.min.js

[bananaappletw]

Support the functionality of generating minified socket.io.min.js.

The kind of change this PR does introduce

• a new feature
• an update to the documentation

Current behaviour

No minified socket.io.min.js.

New behaviour

After running make socket.io.js, minified socket.io.min.js is generated.

Other information (e.g. related issues)

None.

[bananaappletw]

Sometimes the test will pass sometimes not.
Seems like timeout is too short for the test.
If the Travis ci's container is in good condition, the test will pass.
Else not.

[darrachequesne]

I think it related to the node target, which now equals to node v7 (it's not related to the PR).

[bananaappletw]

@darrachequesne
Not exactlly.
It's not related to PR.
Neither Node version.
Take a look of this one.
https://travis-ci.org/socketio/socket.io-client/builds/174189877
Test failed in Node 0.10

[bananaappletw]

@darrachequesne
I think this test prove it.
It passed!

[bananaappletw]

@darrachequesne
Any update here?

[darrachequesne]

@bananaappletw thanks!