Support RegExp expectations

Modify _expect() and _wait() to support both sub-string and RegExp
specifications of what output is expected.
Merge pull request #15 from strongloop/support-regex-matchers

Support RegExp expectations
Improve the error reported by a failed expectation

Modify the error reported when a line does not match the expectation
provided by `expect` or `wait`:
 - use assert.AssertionError instead of plain Error
 - fill actual and expected properties
 - include the expectation in the error message
Merge pull request #16 from strongloop/better-failure-messages

Improve the error reported by a failed expectation