Automatic registration of server env vars

[ZeeCoder]

I implemented the solution we aggreed upon in issue #270.
The problem is, that it breaks an existing test, and I can't figure out why.

Can someone help me out with this?
(If this issue is resolved I'll add tests for the new functionality too.)

Here's the output of phpunit:

PHPUnit 4.6.6 by Sebastian Bergmann and contributors.

Configuration read from /home/zeecoder/work/deployer/phpunit.xml

............................................F.................... 65 / 74 ( 87%)
.........

Time: 2.13 seconds, Memory: 12.75Mb

There was 1 failure:

1) Deployer\Server\BuilderTest::testEnv
Expectation failed for method name is equal to <string:set> when invoked 1 time(s)
Parameter 0 for invocation Deployer\Server\Environment::set('server', Array (...)) does not match expected value.
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'name'
+'server'

/home/zeecoder/work/deployer/src/Server/Builder.php:36
/home/zeecoder/work/deployer/test/src/Server/BuilderTest.php:120
/usr/share/php/PHPUnit/TextUI/Command.php:186
/usr/share/php/PHPUnit/TextUI/Command.php:138

FAILURES!
Tests: 74, Assertions: 208, Failures: 1.

[antonmedv]

It's fail down because of (BuilderTest.php#L115)[https://github.com/deployphp/deployer/blob/master/test/src/Server/BuilderTest.php#L115]
In this test Deployer\Server\Environment expected to be called once. And you add second call in this PR.

[ZeeCoder]

I think it should be okay now.

[antonmedv]

Please, before i merge this PR, add docs to deployphp/docs too. Thank a lot!

[antonmedv]

Now we need set as private here.

[ZeeCoder]

I had to redo this due to incompatibilities with the resent changes.
Also it contained 5 commits for something really simple.