Created EpitopeCollection, refactored effects

[iskandr]

• Created an EffectCollection, which is used for filtering and grouping sets of effects.
• VariantCollection.effects() now returns an EffectCollection.
• Merged functionality of top_effect() and highest_priority_effects into EffectCollection.top_priority_effect()
• Created a base class Collection from which VariantCollection and EffectCollection get their simpler methods like __iter__ and __len__
• Simplified the effect hierarchy, expanded the ambiguous aa_pos (which means slightly different things for different effect types), into the more explicit aa_mutation_start_offset and aa_mutation_end_offset.

Fixes #64 and #58.