jUnit Support

The output looks nice when there are errors, but may be problems when there are not any errors.

I'm not a jUnit expert, so mainly I wanted to some feedback to make sure I was close :)

This will hopefully solve issue #16.

```
~/dev/eslint [master] $ node ./bin/eslint.js ./lib/options.js -f junit
<?xml version="1.0" encoding="utf-8"?>
<testsuites>
<testcase package="eslint" time="0" tests="15" errors="15" name="./lib/options.js">
<error message="Missing semicolon."><![CDATA[line 12, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 18, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 21, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 22, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 23, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 26, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 27, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 30, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 33, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 34, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 35, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 41, col 0, Warning - Missing semicolon.]]></error>
<error message="Missing semicolon."><![CDATA[line 42, col 4, Warning - Missing semicolon.]]></error>
<error message="Unexpected console statement."><![CDATA[line 42, col 4, Warning - Unexpected console statement.]]></error>
<error message="Missing semicolon."><![CDATA[line 45, col 0, Warning - Missing semicolon.]]></error>
</testsuites>
```