Change journal mode to MEMORY.

WAL has some issues if you want to use the database in
a distributed manner, it doesn't work well with multiple
hosts accessing the same database and requires write
access to the host directory for the user using the
database.

https://www.sqlite.org/wal.html has some more details.
Merge pull request #33 from roryk/master

Change journal mode to MEMORY.
add doctest to feature.Attributes
error if file doesn't exist
pragmas can now be handled flexibly and on-the-fly
rm newline from example file
customizing pragmas

- commit them to the db after setting
- create separate set_pragmas() method on creation and interface
- create_db now properly sets pragmas on the returned db
module-level import feature_from_line
sqlite3.OptimizedUnicode default text factory
fix tests to reflect OptimizedUnicode behavior
docs for Attributes
Python3 support + small fixes and circular dependency resolving