Feature n50

Hello Aaron,
If interested, I have added an "NX" function to statistics.py for computing the read N50 (for example) and added some lines to stats.py such that N25, N50, and N75 are all automatically computed with the command "poretools stats fast5dir/". It is currently in a feature branch called "feature_N50" (not the master branch). 
best,
 John Urban (from MAP)