Removes starting slash from request path if present
Necessities for Google App Engine

As GAE filesystem is not writable, then an exception is always thrown
because of the cache folder. But if you already have a cache file before
uploading then not being able to write it is not a problem anymore. So
it is enough to check for write permission in the
HumanReadableCache::set() method.

In the Restler::getPath() method, GAE gave me a notice because $_SERVER
doesn't have an index 'SERVER_PORT'. It is fixed now.
Fixed detecting server port
Merge pull request #289 from kocsismate90/master

Supporting Google App Engine