use safe_repr to print arg vals in trace

This fixes a problem in which extremely long (and slow) stack traces would be produced when
function parameters are large numpy arrays.
Merge pull request #91 from dstahlke/reprfix

use safe_repr to print arg vals in trace
Add more doc / comments on the max_nbytes behavior
ENH: make memory respect mmap_mode at first call too
Merge pull request #95 from ogrisel/memory-mmap-mode

ENH: make memory respect mmap_mode at first call too
ENH: ThreadPool-based backend for GIL-releasing tasks