Initial commit of YAQL:
lexical tokenizer, grammar parser and expression executor of the context-based pluggable functions for basic YAQL operations.
 Implemented basic set of functions:
 - loading of data from context
 - attribute access (including collection iteration)
 - scoping
 - collection filtering
 - comparison operators
 - arithmetic operators
 - boolean operators
 - generation of simple data structures (list, dictionary, tuple)

 started implementations of some advanced functions:
 - predicate-based collection joining
 - namespace-based symbol resolution and validation
 - collection-to-collection conversion by using "select" syntax
 - Random number generation
 - "Take-while" syntax
 - summing of collection values