Fix for DST issues in master branch (issue #810)

Backport of
https://github.com/brutasse/graphite-api/commit/c8854f584b4ba216f9706fa5bf42597a4f9c969c
https://github.com/brutasse/graphite-api/commit/a0ae47023cce8fe7dc2815c8ed46ffc16f3bdefe
to master.
Oops, wrong epoch conversion
Right (tm) test provided, fix in attime also backported
Finishing touches, fixing import warnings
Merge pull request #991 from deniszh/master_dst_fixes

Master DST fixes