Translate improvements

[jgosmann]

Motivation and context:
This PR makes a couple of improvements to the vocabulary translation mechanism.

• Missing keys in the target vocabulary will be properly ignored when populate=False. This fixes Add argument to ignore missing keys in translate #56.
• Adds a solver argument proposed by @arvoelke that can give slightly better transformation matrices.

To make it possible to use the solver argument in action rules, I imported the whole nengo name space into the action rule built-ins. But it occured to me that this cannot be the final solution because solvers might be defined outside of nengo and these solvers should be usable to. However, making all globals accessible is also problematic because names can refer to modules or semantic pointers and at parse time this can currently not be determined (technically it should be possible, but will require some major changes to the parsing). Maybe the best option is to have the user explicitely populate the builtin namespace, but this will increase boiler plate code.

Interactions with other PRs:
none

How has this been tested?
added tests

How long should this take to review?

• Average (neither quick nor lengthy)

Where should a reviewer start?
Might make sense to do this one in order of the commits.

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• [n/a] I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• All new and existing tests passed.

Still to do:

• Figure out a better and more general way to provide access to solvers in action rules.