server.php: Add an API for accessing variables

We don't yet have an API for accessing _SERVER variables, the reason
being that we have full control over which variables that we
access. However, the ability to call pip_server_get() is still useful
since it returns an empty string if the variable is not set, preventing
an Exception being raised about attempting to access an invalid variable
index.

Issue #207.
history.php: Add a function for current URL

This function constructs the full URL of the current page, and is the
functional equivalent to JavaScript's window.location variable.

Issue #207.
s.php: Fix pagination URL construction

This makes the function that constructs paginated URLs smarter, by
setting or replacing the 'start=<num>' GET variable in the query string
of the existing page, rather than constructing an entirely new URL.

Issue #207.