Add named constants for P4Ruby's exception levels.
The valid levels
are:
P4::RAISE_NONE -- No exceptions
P4::RAISE_ERRORS -- Exceptions on errors only
P4::RAISE_ALL -- Exceptions on errors and warnings
Also added P4#at_exception_level( level ) { ... } method to allow you
to run a block of code at a different exception level and revert to
the previous level when the block completes. Thanks to Johan Nilsson.
Some doc tidying along with the docs for the features above.