Added verification for at least number of times

When stubbing servers for integration tests, there are scenario's where
it is unknown how often the server is invoked (e.g. when using a timer
trigger). To aid this use-case, the atLeast() method was added, so that
the test passes also when the server is invoked multiple times.
Merge pull request #18 from mdonkers/master

Added verification for at least number of times