Add integration test framework

This has a lot of boiler plate to get it setup the the end result for people needing to write integration tests to test the libcontainer API is fairly straight foreward.  It can still be refined to make writing these tests easier but this should be a good start in building out our own test suite. 

The end result is in `integration/exec_test.go`
typo newStdBufffers --> netStdBuffers