Multi variable binning

Completing issue #91 + some minor fixes:
 - fixed missing initialisations in constructor
 - added event variables M3, M_bl and angle(b,l) to Event class
 - added multi-fit-variable for Binned_variable_analyser including correlations
 - replaced 1-fit variable analysers with multi-(fit-)variable analysers in TTbar_plus_X_analyser

Old functionality is preserved (just handled differently internally).
Please test and be in awe (or not).