Update TODO.txt

VCF extension?
Update Makefile

Git history anachronism corrected.
Cleaned up the integrator functions, added minI and maxI
Merge branch 'master' of github.com:Ensembl/WiggleTools
Handling absence of data in apply function
Adding default values to Pearson calculation
Update TODO.txt
Correcting possible infinite loop in power function
Adding better detection of NAN
Updating online help to include VCF/BCF files
Removed useless function pointers to multiplex and multiset pops
Added strict multiplexers
Transformed ratio into strict multiplexer
Adding default operator
Making default_value a required parameter
Handling NaN as input/output of iterators
Cleaning up header
Added optional strict multiplexer in command line
Small optimisation for strict multiplexers
Allow the printing of nan as they are accepted by the kent apps