Work through several open todos:

Work through several open todos:
* Added test for pickling contexts
* Renamed ExceptionList to Signals (to match wording in the spec)
* Simplified Context constructor by allowing flags=None to automatically
  generate a zeroed-out flags dictionary.
* inlined _convertString() which was used only once
* _rounding_decision is private, so excluded its contants from __all__.
* added an XXX comment with concerns about subclassing signals results in
  a deviation from the spec (maybe important, maybe not).
* Taught the test_suite to determine its own directory (modeled after code
  in regrtest.py).  Enables it to be run when the current directory is not
  the test directory.
* Added a clear_flags() method to the Context API to make it easier to do
  a common operation with flags.
* Fixed the trap_enablers defaults in BasicDefaultContext to match the spec.