Bug Fixes and add Multiple Authors Improvements

#### When generating the extension xml file, the absolute path of the temp directory is being included `<path>` node value. This pull request resolved this bug.
 
Example: 
```xml
<_>
	<contents>
		<target>
			<target>magecommunity</target>
		</target>
		<path>
			<path>Package_Name/Module_Name/Model/SomeClass.php</path>
		</path>
		...
	</contents>
</_>
```
The `<path>` node uses the absolute path including the `tmp` directory where the file first got generated.
#### The Magento connects extension has a known bug where the first contents nodes, target, path, type, include, and ignore nodes in the extension xml needs to be emptied otherwise when generating the package the first file will not be included.

Example: 
```xml
<_>
	<contents>
		<target>
			<target></target>
			<target>magecommunity</target>
		</target>
		<path>
			<path></path>
			<path>Package_Name/Module_Name/Model/SomeClass.php</path>
		</path>
		<type>
			<type></type>
			<type>file</type>
		</type>
		<include>
			<include/>
			<include/>
		</include>
		<ignore>
			<ignore/>
			<ignore/>
		</ignore>
	</contents>
</_>
```
Adding these empty nodes `<target></target>`, `<path></path>`, etc will solve the missing file issue with Magento Package extension.
#### Fix the unnecessary extra `value` node when generating extension xml file content
 
This bug was introduced when the code was adding value to a `SimpleXMLElement` object using the `value` property, this instead added the node `value` and the content that was assigned to it.

Example:
```php
$xml = simplexml_load_string('<_/>');
$xml->addChild('example')->value = 'This is a test';
``` 
The example above will produce this xml:
```xml
<_>
	<example>
		<value>This is a test</value>
	</example>
</_>
```
but the intention was to produce this:
```xml
<_>
	<example>This is a test</example>
</_>
```
#### Add multiple authors simply by adding comma delimited name, user, and email in the extension-config.php file

Example:
Configuration:
```php
return array(
	...
	'author_name'  => 'Name 1, Name 2, Name 3',
	'author_user'  => 'user1, user2, user3',
	'author_email' => 'email1@someplace.com, email2@someplace.com, email3@someplace.com',
	...
);
```

Result:
```xml
<_>
	<authors>
		<name>
			<name>Name 1</name>
			<name>Name 2</name>
			<name>Name 3</name>
		</name>
		<user>
			<user>user1</user>
			<user>user2</user>
			<user>user3</user>
		</user>
		<email>
			<email>email1@someplace.com</email>
			<email>email2@someplace.com</email>
			<email>email3@someplace.com</email>
		</email>
	</authors>
</_>

```