Add support for linked files in projects.

When a file from outside the project directory is added as a Linked File
OctoPack would generate an incorrect source and destination path in the
.nuspec file and as a result the file would not be included in the
package. This adds support for that scenario.
Add unit tests for linked file support.

This adds an Nunit based unit test project and tests for the linked file
support.
Merge pull request #16 from jeff-french/master

Add support for linked files in projects.