Add URL class.

Adds an `Url` class which is (currently stub) object representation of
an URL. Also moves URL decoding utility method from `Utils` class to the
new `Url` class in an attempt to make `Utils` class slowly obsolete.
Move escaping methods to String class.

My opinion is that this is the most logical place for them. Also we can
slowly move methods out from `Utils` class and make it obsolete.
Merge pull request #81 from peelonet/string-changes

URL class and escaping methods