Issue #132 Proper googletest dependency and fixes for unitests on OSX.

Problem:
It's really hard to get Libuavcan tests building on a mac or other platform if googletest isn't installed.

Solution:
add "if linux" guards to problem areas. Also include googletest as recommended in the googletest project's README.

Testing:
Successfully built on OSX sierra using gcc6

.gitignore

@@ -17,6 +17,9 @@ __pycache__
 .pydevproject
 .gdbinit
 
+# vsstudio code
+.vscode
+
 # libuavcan DSDL compiler default output directory
 dsdlc_generated