* 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.
