Improvement: Replaced "priority" with TokenType
* Made implementation more expressive by replacing the integer priority with an strictly typed enum called TokenType
** Made removal of character comparisons from tree construction and lexer possible
* As a side effect distinct numbers had to be assigned to each token
** Operators of same priority do not have identical numbers anymore
Merge branch 'master' into feature_constants

Conflicts:
	src/tree.cc
	src/utils.cc