Reorganise tests
Add -deps flag; default -exclude-goroot to false

The new -deps flag (false by default) determines
whether or not to recursively instrument package
dependencies. With the introduction of this flag,
the default value of -exclude-goroot has been
changed to false.

Fixes #26
Merge branch 'master' of https://github.com/axw/gocov
Document -deps, -exclude-goroot changes