Add python2.6 compatibility
OrderedDict is imported from pycompat

OrderedDict exists in ordereddict only in Python2.6.
Re-write 2.6-incompatible dict comprehensions
Fix more incompatibilities, i.e. set vs {}
Update travis matrix for 2.6
Use unittest2 rather than unittest
Use unittest2 only for python2.*
Further incompatibility fixes and mods

* install and use unittest2 for 2.6 tests
Fix .travis.yml typo
Fix concat method of DataArray