REL: updates based on comments from @ElBrogrammer and @wasade
MAINT: removed top-level imports since we hope to have a high-level wrapper for these in the future, see #474
MAINT: updated imports
DOC: edits
DOC: added references for BLAST and BLOSUM
DOC: updated parameter descriptions
MAINT: updated to futures version of zip and range
MAINT: removed unnecessary lines
ENH: updated to retain sequence ids, if they're provided with input
MAINT: bug fix
MAINT: expanded tests
MAINT: changed type based on comment from @wasade
MAINT: added spaces for readability
MAINT: updated list indexing format
MAINT: updated for clarity
MAINT: updated so more informative error message is printed if characters are in sequences that are not in the substitution matrix
ENH: updated to better match SSW interface, thanks @ebolyen