Modify the handling of @ symbol when specifying the check range

It looks like the @ symbol does not behave as defined in http://nagiosplug.sourceforge.net/developer-guidelines.html#THRESHOLDFORMAT.  In the range code the @ symbol changed the range 10:20 from alerting on x < 10 or x > 20 to alerting on x <= 10 or x >= 20, when it should have change it to alerting on x >= 10 or x <= 20.

Because the spec is a bit difficult to understand, I generated this table.
Range   Generate an alert if x
10      x < 0 or x > 10, (x does not alert if x >= 0 or x <= 10)
@10     x >= 0 or x <= 10, (x does not alert if x < 0 or x > 10)
10:     x < 10, (x does not alert if x >= 10)
@10:    x >= 10, (x does not alert if x < 10)
~:10    x > 10, (x does not alert if x <= 10)
@~:10   x <= 10, (x does not alert if x > 10)
10:20   x < 10 or x > 20, (x does not alert if x >= 10 or x <= 20)
@10:20  x >= 10 and x <= 20, (x does not alert if x < 10 or x > 20)

So, with the above information, I changed the specs to match the above table.  Then I went and modified the range code to handle the change in the specs.
Merge pull request #2 from shr3kst3r/master

Modify the handling of @ symbol when specifying the check range