v1.5.14 Compartmentalize Biostrings import with importFrom, importClassesFrom

Still some warnings from class import. R doesn't like the missing
subclasses when SetClassUnion() is called...
1.5.14 Fixed class import warning for importing XStringSet subclasses
Merge pull request #206 from joey711/biom-pkg-depend

v1.5.14 compartmentalize Biostrings with importFrom and importClassesFrom