Fixup getURLParameter behavior

This commit further changes the behavior of getURLParmeter to handle
encoded parameter values and returns the decoded string and improves
behavior for parameters without value and multiple equals and other
similar cases which did not work before. See the comments at
http://stackoverflow.com/questions/1403888/get-escaped-url-parameter for
a list of the issues handled by the updated implementation.

This change does not change the general behavior of the function. Empty
or non existing parameters still return an empty string ('').