Fix rename interval SEGV

.. when called from popupMenu.
LTM tooltip for groupby weeks is misleading

.. it lists start date for this week to start date for next week
.. now changed to say 'Week commencing <date>'.

Fixes #501.
Fix custom date range muddle when groupby weeks

.. the start date would progressively change as you selected a
date range in the sidebar. This is because the code to set the date
range to the start of a week looked at the wrong date variable when
using a custom date range.

Fixes #500.
Added notes on the build

Basically pointed out you need QT 4.8 minimum and that use newer
libs if they exist from Windows except for libusb (0.1.12 required)
Merge branch 'master' of git://github.com/GoldenCheetah/GoldenCheetah into upstream_master
Merge branch 'upstream_master'