Continuous integration for lmfit-py on travis-ci

lmfit should have unit tests (see issue #2) and we should use travis-ci to make sure the tests pass and docs build for Python 2 and 3 using the free continous integration service https://travis-ci.org .

Also see https://github.com/newville/asteval/pull/4

Here's a build where running a simple test script worked:
https://travis-ci.org/cdeil/lmfit-py/jobs/3220941

This is not ready to be merged, we need unit tests first ( issue #2 ) and I have to fix the installs of the dependencies for the sphinx build and actually use Python 3 sphinx for the Python 3 build.

https://travis-ci.org/cdeil/lmfit-py/builds