DOC typo
FIX + COSMIT Reuters out-of-core example

* download to scikit_learn_data, not pwd
* copyedited the docs
* rewrote a class as a simple function
* use glob instead of os.walk, the directory structure is flat
* trim output to fit in an 80-column terminal window
* changed some identifiers

XXX there's a bug in the minibatch splitting: I only get 973 test
samples, while a 1000 are requested:

Test set is 973 documents (125 positive)
100 train docs (    14 positive) accuracy: 0.840 in 1.09s (   91 docs/s)
1067 train docs (   166 positive) accuracy: 0.918 in 2.28s (  467 docs/s)
2027 train docs (   310 positive) accuracy: 0.925 in 3.36s (  603 docs/s)
...