Expression support boolean to fix #152

[lixingwang]

What kind of change does this PR introduce? (check one with "x")

[*] Bugfix
[] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:

Fixes: #152
What is the current behavior?
Does not support this case: string.equals("sea", "sea") == true or isDefined($activity[rest_2].result.photoUrlz) != false
What is the new behavior?
Support expression like: isDefined($activity[rest_2].result.photoUrlz) != false or string.equals("seafood,name", "sea") == false