Run part of travis on Python 3

- fix install of pydot by using the right version of pyparsing on python 2
- pydot does not run on python 3 for the moment
- run tests on the installed files, not the source

Add DictMixin/OrderedDict to theano.compat in PY3.

Reordered for ease of reading (same order in if/else).

Fix test, compile less, doc, add tests.

Improve compile time on slow filesystems

Scan opt to push out Dot products outside of Scan's inner graph

CRASH/OPT fix (quick to review)

Removed comparisons with None

Fix broadcast pattern of dnn conv op.

Revert change that break too many test in floatX=float32