Added initial struct for Index.
Added initial struct for Index.
Basic single column index works. No API yet though.
Merge branch 'index-support' of github.com:jeromekelleher/vcfdb into index-support

Conflicts:
	_vcfdbmodule.c
Fixed multi-column and variable length indexes.
Working RowIterator using index with min and max.
Added callback for index build.
Added test cases for indexes and found int bug.
Fixed integer bug and bumped schema version.
Added a bunch of index tests.