Refactor, increase error coverage, more sanity checks.

Refactor, increase error coverage, more sanity checks.

* Deal with when source is a binary for test scripts.
* Move chdir from command to util module
* Reclassify util.chdir exceptions to be subclasses of IOError
* Migrate forces earlier (for the sake of better code)
* Deploy scripts now have verify methods which throw exceptions
  on event of errors.  This includes newly implemented
  DAG validation (verifyGit)
* Compactify some open().read()/write() calls
* Add tag property to ApplicationVersion
* Change shell.drop_priviledges() API to require a directory
  as first parameter.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>