export/hazard:

Added handling for the case where the GMPE logic tree contains two
branching levels. In this situation, the export dir is a concatentation
of the GMPE names, joined by _.
tests/export/hazard_test:

Added a test for the error described by
https://bugs.launchpad.net/openquake/+bug/1098518.
Merge pull request #1000 from larsbutler/fix-gmpe-export-dir

Fix result export with multiple branching levels

[r=gvallarelli] [f=1098518]