conda run executes programs in /bin as well; fixes #803

Fixes #803.

TODO

- [x] On Windows include `%PREFIX%` as well (as python isn't in `\Scripts`)


```
lidavidm@David-Arch ~ % conda run python
Fetching package metadata: ...
Python 3.4.1 |Continuum Analytics, Inc.| (default, May 19 2014, 13:02:41) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
lidavidm@David-Arch ~ % conda create -n old python=2.7
...
lidavidm@David-Arch ~ % conda run -n old python
Fetching package metadata: ...
Python 2.7.8 |Continuum Analytics, Inc.| (default, Jul  2 2014, 18:08:02) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
Anaconda is brought to you by Continuum Analytics.
Please check out: http://continuum.io/thanks and https://binstar.org
>>>
```
Good point. I do want to know whether conda failed.