Added function requiresVocabulary() for DescriptorsType
Added class Builder
Removed deprecated method
Added missing else branch
Removing deprecated warning
Fix SIGILL

Actually only using DescriptorsType& causes error.
Later, when optimizing the code we should remove redundant/unused
attributes and use references/pointers whenever possible.
Added documentation to ImageSearch class
Splitted test_imsearch test into load and build tests

test_imsearch_load just loads data from files, test_imsearch_build
actually recompute descriptors and reindex images. For this reason the
latter is disabled in CMakeLists.txt but can be manually executed.
Anyway, these are actually integration tests and should be executed at
a later stage, not for during unit testing.