Allow absolute paths in Environment::path()

[thekid]

Unchanged behavior with relative paths:

$ xp -w 'use web\Environment; (new Environment(getcwd()))->path($argv[1])' src/main/php/
./src/main/php/

Changed behavior with absolute paths:

# Before
$ xp -w 'use web\Environment; (new Environment(getcwd()))->path($argv[1])' /etc
.//etc

# After
$ xp -w 'use web\Environment; (new Environment(getcwd()))->path($argv[1])' /etc
/etc

See #124 (comment)