URL class and escaping methods

1. 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.
2. Move XML and JSON escaping methods from `Utils` class to `String`
class which in my opinion is the most logical place for them. Also renames
JSON escaping method to `EscapeJavaScript`.