Split libraries fastq

Adds the StudyPreprocesser class, which allows to preprocess the raw data of a study. Currently, it only supports Illumina data, but the code should be easy to expand to support other types.

I've also modified the `insert_filepaths` function, so it now supports to pass the filepath type either by id or by the string (name).

I've modified the initialize.sql file, to support the correct filetypes for the Illumina data and its preprocessed data. I had to modify multiple files due to this change.
This is the output of QIIME, they are added to the DB once the PreprocessedData gets added to the database. The output directory for the split_libraries_fastq.py is currently generated using mkdtemp. If we want to save all these intermediate results, yeah I agree that we need to specify some directory (working directory) on the QIITA config. What others think?