#43 a tool for checking overall coverage

[Vlad Roubtsov]

This is somewhat against EMMA spirit (which encourages
interactive browsing of coverage reports instead of
letting an automated build decide whether coverage is
Ok), but other coverage products seem to have this so
we should add this to stay competitive.

The tool should allow users to form a composite boolean
expression to be checked (class coverage > 95% and
method coverage > 80% and line coverage > 70%, etc).
Not clear whether it should allows ORs at this point.

Another thought here: allow such boolean checks to be
applied at package levels as well, not just "all classes".

The ANT version should set a user-provided property (or
optionally fail the build as another convenience) when
the boolean expression is false.

The command line version should probably return a
special exit code (with -exit).