adding tests including SequenceCollection of length 0
bug fixes related to empty alignment objects
added failing tests when sequence_count == 0
fixed failing tests
improved names of test variables
diplomatically handled @ElBrogrammer's power trip
added is_empty method and tests
updated code to call is_empty, rather than test for emptiness directly
one more call to is_empty()
Merge pull request #258 from gregcaporaso/issue-194

additional tests and some minor bug fixes in the SequenceCollection and Alignment objects