Paired-end assembly app controllers.

Here is the code I've added for paired-end joining application wrappers.

code:
cogent/app/fastq_join.py
cogent/app/flash.py
cogent/app/pandaseq.py
cogent/app/seqprep.py

test code:
test_app/test_fastq_join.py
test_app/test_flash.py
test_app/test_pandaseq.py
test_app/test_seqprep.py