User comment cannot contain colons.

The useradd command quits with the following:
```
useradd: invalid comment 'Team City Agent: ag1'
```

It works without colon.
correct not_if check for download/unzip, closed #4

This commit fixes #4 (Fix bug that prevents unzip from running when zip file
exists). This check was designed for the remote_file download.
Additional, the explicit check is replaced by the chef action :create_if_missing
stop on agents without server_url - closes #1
Merge pull request #2 from jgraichen/fix-user-comment

User comment cannot contain colons.
don't use activesupport methods like present?
skip agents with nil instead of attribute hash

To support previous defined agents (e.g. the default agent from cookbook
attributes), the hash can be replaced by nil. The recipe ignores these
agents.
correct group of teamcity installation
optimize agent source download behavior

* do not remove .zip per cookbook (cache-directory cleaning is task of chef)
* use md5 digest of server_url instead of agent name to avoid source misusages
support agent starting and create init.d service

* recommends java to install java jdk
* create init.d service scripts for the agents
readme: add missing code end marks (`)
use agent names only if there is more than one agent

Remove old default attribute of agent. Count all agents and use agent names
only if it is needed to distinguish between more agents.
Add message when downloading build agent
Fix template on nil properties
Merge branch 'master' of github.com:mswart/chef-teamcity

Conflicts:
	recipes/agent.rb