ENH add latent semantic analysis/sparse truncated SVD

Fixes #1519.
ENH use rnd SVD in TruncatedSVD by default for speed
COSMIT omit unused parameter/return value in svd_flip
TST strengthen TruncatedSVD tests
DOC + MAINT deprecate RandomizedPCA scipy.sparse support
FIX and link LSA clustering example
DOC explain normalization in LSA KMeans example

As requested by @ogrisel.