Letor stabilisation - project progress

Hello everyone,

I have completed introducing some code from v-hasu's branch into mine,
mainly for Features, FeatureVector and FeatureManager classes. I have
pushed the changes to https://github.com/ayshtmr/xapian/tree/letor-update. I
am now proceeding to write unit tests for feature modules.

There are a few things that I wanted to clarify:

1. I have introduced a lot of code in a single commit (
https://github.com/ayshtmr/xapian/commit/2abaf6b8d64b1624699c3f490b6dc441e1c6cdf3
)
Is it fine or should I try to split things up?

2. There is a lot of commented code in my branch right now that needs to be
removed. Is it wise to take care of it with each commit I push or should it
be done later, say, when some stability is reached (post mid-term?)

Regards,
Ayush