Fix: Removed errorneous internal parameter comment

Introduced in error in 4281a9ebfa6fb2dc40197832de64bdeb957814da
Fix: Constructor @return type-hint not needed
Fix: Path to Net_Url2.php

Path contained superfluous dot.
Fix: PHP 5.2 compat path to Net_Url2.php

PHP 5.2 compatibility
Fix: Minimum required PHP Version is 5.1.0

Net_URL2 makes use of class Exception which requires 5.1.0.
Imp: Composer package description and tags
Readme as markdown

The plain text README has been turned in markdown as it is intended to be read on Github.
New: Init Travis CI

- .travis.yml added, phpunit
- Added status image to README.md