add binwidth option

binwidth of 0 automatically resets to 0.1
change existing variable bin_width to binwidth to match bincount
modify bin calculation to include the max data value

![cmd](https://cloud.githubusercontent.com/assets/529129/2574709/5cec5cd2-b939-11e3-89b1-6faa0c0ecac0.png)
![before](https://cloud.githubusercontent.com/assets/529129/2574704/4574dbe2-b939-11e3-9160-6fd8cbf39233.png)
![cmd_with_w](https://cloud.githubusercontent.com/assets/529129/2574729/a4d9267e-b939-11e3-8dec-c2229ea9394d.png)
![w100](https://cloud.githubusercontent.com/assets/529129/2574712/72294d94-b939-11e3-8805-7b7bdbfdd7c3.png)
![cmd_with_w50](https://cloud.githubusercontent.com/assets/529129/2574748/e5aa7220-b939-11e3-9976-48e34b5ea435.png)
![w50](https://cloud.githubusercontent.com/assets/529129/2574715/75bbd4cc-b939-11e3-9c02-4f248d5a96f5.png)
![cmd_with_w10](https://cloud.githubusercontent.com/assets/529129/2574744/c93eafb6-b939-11e3-8528-83453278721b.png)
![w10](https://cloud.githubusercontent.com/assets/529129/2574719/79aca94e-b939-11e3-9ba3-d6b1661a28fb.png)