better indels
Complete refactoring of SNP filtering. It is now much easier and coherent. SNP filters are now subclasses of SNPFiltering.SNPFilter with a function "filter(self, chromosome, **snpSets)". The filter function must return either a SequenceSNP, a SequenceInsert or a SequenceDel. The genome takes an instanciated object as parameter to the 'SNPFilter' argument  allowing for much more user customization.

+ added:
 * tests
 * examples
 * more documentation