Add support for external modules
Install modules and improve module load error message
Add more units tests to 'require'
Improve grammar modules

- return a table instead of just the grammar
- add time_ns utility function
- add additional unit tests
Test condition and return (reduce indenting)
Add a package cache for require
Protect against a 'Fields' key having a nil value during protobuf serialization
Backout the nil check

The problem was due to a "Fields" member containing an empty table.
The is a mal-formed message structure and should fail injection.
Set TZ to UTC on sandbox creation.
Merge pull request #1 from mozilla-services/grammars

Grammars