Added test harness and Raindrop constructor tests

A test harness was added, and tests included to make sure the public
Raindrop constructor detects null and empty parameters.
Changed ViewDataDictionary to IDictionary

Use of ViewDataDictionary was changed to IDictionary<string, object>.
The IDictionary interface is more generic, allowing more varieties of
data source to be used.

Several unneeded using statements were also removed.
Minor TagStream refactoring

- Changed "FilePath" to "Name" to reflect the fact that a stream doesn't
have to come from a file.
- Moved some members of the Settings object into the TagStream in
preparation for making the TagStream handle all syntax.
- Corrected a comment that claimed the opposite of the truth.