Refactor RSpec test task.

We can now run specific tests by appending items to the task name.  For
example, `buildr rspec:vcpu,consumer` will run any spec file that begins
with "vcpu" or "consumer".  We can exclude tests with a minus sign in
front of the identifier.  E.g. `buildr rspec:-vcpu` will run all spec
files that do not begin with "vcpu".

Additionally, we can provide either line numbers or test names to the
task.  For example, `buildr rspec:vcpu:62` will run the test on line 62
of the vcpu spec file.  Likewise, `buildr rspec:vcpu:consumer` will run
all tests in the vcpu spec that have the word "consumer" in the test
name.

Please note that if you need to use a phrase to single out a test, you
will need to quote the task name: `buildr "rspec:vcpu:should be valid"`.
Also note that any phrase you specify will be applied to *all* tests.
So `buildr "rspec:vcpu,consumer:is valid" will require tests in both the
vcpu spec and consumer spec to contain the phrase "is valid".  This is a
limitation of rspec itself.