[0.1.dev18] Installation improvements and continuous integration

[smmaurer]

This PR improves installation and testing.

1. Moves requirements out of setup.py and into their own file(s). This allows a single list of requirements to be used for pip environments, conda environments, and in the setup script.

2. Divides dependencies into (a) minimal requirements for model management and core templates, (b) additional requirements for less-used templates, and (c) requirements for development and testing. This is a common design pattern (e.g. pandas uses it) to avoid dependency bloat, keeping installation as fast and reliable as possible.

3. But this means we need to help out users who might be missing dependencies for certain template functionality. Following pandas, I've moved the relevant import statements inside try-catch blocks that raise clear errors telling users what's going on and how to resolve it.

   (We can use the same strategy for imports that often have version compatibility problems, like the choicemodels development releases that are currently required by the Large MNL template.)

4. Adds a Travis script to automatically run the test suite across python versions when commits are pushed to Github.

5. Incidentally, it seems like the templates work fine in Python 2.7 -- i hadn't tested this until putting together the Travis builds.

6. Updates the README. More substantial documentation improvements coming separately.

Versioning

Updates the library version to 0.1.dev18

Before merging:

• fix import statements for pylogit
• review other dependencies
• update version
• coveralls messages for the PR thread