De (Dynamic Expression) module provides means to build and evaluate
dynamic expression of arbitrary complecity and operands/operators nature.

Expression is considered as tree consisted of operands and operators.
Operater must have child nodes. Operand is terminal node.

Tree example:
   (a + b) * c + d

   - +
   --- *
   ----- +
   ------- a
   ------- b
   ----- c
   --- d

   Here "+" and "*" are operators,
   "a", "b", "c" and "d" are operands

Expression tree structure is built with the help of <rubytree> gem
(https://rubygems.org/gems/rubytree)

In addition to basic classes of Expression, Operand and Operator
module provides some extensions for different nature expressions:
  - Boolean expressions
  - Sunspot Solr search expressions