Major reorganization of internal modules

Most of xray -> xray.core. xray.backends -> xray.io.
Remove circular dependencies from xray.core
Move tests to xray/test and update setup.py
All relative imports in xray.io
Move squeeze from utils to common
Split formatting off from common.py
Split off new core.alignment module
remove unnecessary compat in test/test_io.py
Update .travis.yml

Test 3.4

Make the pydap tests optional
Move io back to backends
Reorder pydap test to the end so it does not block other tests
use conda instead of pip to install unittest2
Merge pull request #216 from shoyer/reorg

Internal code reorganization; creates xray.core module