Store generated files in a temporary directory created at import time

/cc @volpino 

We're using plyplus in a django project and we use linters to check the source quality. Pylint, specifically, lacks the feature to ignore files based on regular expressions, so there's no way to exclude the grammar's generated files.

Moreover, as we use jenkins for testing our application, there's no way to know if the current directory is writable or not (actually, the same issue may be valid in production, using supervisor for running the process).