Add validation to `trackUnstructEvent`

[alexanderdean]

Original issue: snowplow/snowplow#209
Author: https://github.com/alexanderdean

Including:

• Checking for null name and properties
• Checking for nesting
• Checking for arbitrary type suffixes
• Checking type suffixes match JavaScript types

[paulboocock]

Moving away from validation in the tracker. We are however considering a debug mode that may have the capabilities to do this: #215