Added support for cache size > 2000

[dalmia]

Fixes #80

This PR replaces long int with long long to support cache_size>2000.

Also, mentioned at scikit-learn/scikit-learn#8012

[TCharnley]

If the compiler being used is DevStudio don't the supporting/calling methods need to be modified as well. int get_data(const int index, Qfloat **data, int len); The int data type in this instance is still 4 bytes. To access additional cache size these also need to be 64 bit data types (--int64 / long long)
Does this effect building a 32 bit version with DevStudio?

[dalmia]

@TCharnley Yes, this is indeed a confusion for me too. I think we should wait for someone to answer that.

[amueller]

Shouldn't this be size_t #80 (comment) ?