Add simple string conversion and match expressions

[ayh20]

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

[] Bugfix
[X] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:
``` Adds a new functions for use in mapping expressions:   
string.tostring(nnn) returns a string  
string.match(expression,string) returns a bool

**Fixes**: # 

**What is the current behavior?**

**What is the new behavior?**

[ayh20]

@mellistibco What needs to be done to get this request merged ?