API: added Table.subsample
DOC: updated how ValueError is thrown
Merge branch 'master' of github.com:biocore/biom-format into issue_125_sample

Conflicts:
	biom/table.py
	tests/test_table.py
API: removed replace, thanks @josenavas
ENH: cython'd subsample
Merge branch 'master' of github.com:biocore/biom-format into issue_125_sample
ENH: cythoned
Merge branch 'master' of github.com:biocore/biom-format into issue_125_sample
BUG/DOC: addressing @Jorge-C's comments
DOC: addressing @ElDeveloper's comments, thanks\!
Merge pull request #397 from wasade/issue_125_sample

API: added Table.subsample