Fix Python 2.6 in timerange and add tests

I have also removed the julian_date flag from time range
because as far as I could tell it was not working.
Allow vso.attrs.Time to be created with a TimeRange object.
Fix docs
add test of creating a timerange from a timerange
Test new vso.attrs.Time arg parsing
Tidy up TimeRange and vso.attrs tests
back to full cov on timerange
Move new tests
tweak to docstring