Lint config file, make sure we don't push broken configs

This diff adds a linter for config files. Included is a test config - I'll follow up with proper test and pre-commit hooks to make sure we don't break it. 

The linter checks for:

- Node names should be a single word
- Every node should contain key-value pairs separated by one `=` sign

Tested locally with the test file and with good and bad config files on Heroku. 

Good file (config at the bottom of http://www.hhvm.com/blog/?p=113, unmodified):
```
-----> Fetching custom git buildpack... done
-----> PHP (HHVM) app detected
-----> Vendoring HHVM 
-----> /app/tmp/repo.git/.cache/hhvm exists, not downloading
-----> Vendoring binaries into slug
-----> Sourcing config.hdf
-----> Found custom config.hdf, linting
-----> Aborting - can't push bad config
-----> Discovering process types
       Procfile declares types      -> (none)
       Default types for PHP (HHVM) -> web

-----> Compiled slug size: 21.2MB
-----> Launching... done, v17
```

Bad file:
```
-----> Fetching custom git buildpack... done
-----> PHP (HHVM) app detected
-----> Vendoring HHVM 
-----> /app/tmp/repo.git/.cache/hhvm exists, not downloading
-----> Vendoring binaries into slug
-----> Sourcing config.hdf
-----> Found custom config.hdf, linting
-----> Unexpected node name in  on line 9
       Foo Bar {
-----> Bad config key-value pair in  on line 15
       File = /var/log/hhvm/access.log = illegal
-----> Bad config key-value pair in  on line 26
       NoValue
-----> Aborting - can't push bad config

 !     Push rejected, failed to compile PHP (HHVM) app
```

With: 

```
Server {
  DefaultDocument = index2.php
}

Eval {
  Jit = true
}

Foo Bar {
  Level = Error
  UseLogFile = true
  File = /var/log/hhvm/error.log
  Access {
    * { 
      File = /var/log/hhvm/access.log = illegal
      Format = %h %l %u %t \"%r\" %>s %b
    }   
  }
}

VirtualHost {
  * { 
    Pattern = .*
    RewriteRules {
      dirindex {
        NoValue
        pattern = ^/(.*)/$
        to = $1/index.php
        qsa = true
      }   
    }   
  }
}

```