[IMP] unwrap tests suites during module loading, prepare for storing/filtering attributes
[ADD] hooks to enable/disable tests running during install

also add flag for post_install run, but not used yet
[FIX] run_unit_tests lying about its return value: it only ever returns a bool, never None
[IMP] slightly improve error message at the end of a module's failed tests
[IMP] phantomjs runner

* use Skip exception to skip tests in case phantomjs binary not found
* remove spurious logging, move some to debug when debatable
* use testing assertions for correct reporting
* allow failure message
* use mutable buffer to accumulate stdout data