Add limited support for libraries during unit test compilation

[hlovdal]

Issues Fixed

• Partially resolves issue 13.

[ianfixes]

This is a feature I can't accept changes on without unit tests, which in this case would involve new SampleProjects that pull in external libs. There are at least 6 use cases that need to be supported, so demonstrating any one of these would be an improvement over what I have now:

• Arduino 1.0.x compatible where all necessary files are in the root directory
• Arduino 1.0.x compatible where some necessary files are included from a subdirectory
• Arduino 1.0.x compatible where some subdirectories contain files that should not be included
• Arduino 1.5 compatible where all necessary files are in src/
• Arduino 1.5 compatible where some necessary files are included from other directories
• Arduino 1.5 compatible where some subdirectories contain files that should not be included

I'm updating #13 with these cases.

[ianfixes]

I'm working on some changes in this area, which might supersede the changes you're making here. Do you have any example projects that are built using these changes? I'd like to add tests against those.

[ianfixes]

I think #61 covers all of this and more. If not, please re-open!