Removed some dead broken code

The compiler does not have a getVariableCompiler method anymore. The
VariableExpression is handled by a type compiler.
Removed the unused itemType property in the compiler

There is no way to write in this private property so the isset check would
always return false.
Thus, this feature was flawed as it would not handle inheritance (checking
for parent classes or interfaces for instance).
Moved the preconditions of function arguments before the function

This allows to call compileInternal on arguments when compiling
preconditions of the function as the preconditions should be compiled
before compiling the expression.
Merge pull request #108 from stof/fix_function_preconditions

Moved the preconditions of function arguments before the function
Merge pull request #109 from stof/dead_code

Removing some dead code
Added a check for invalid variable names.

Fixes #99
Merge pull request #110 from stof/variable_check

Added a check for invalid variable names.
Added configurable roles to be ignored in IddqdVoter