**Current state:** a user invokes `ipython` and is provided with the IPython instance regarding the `$PATH`.
**Proposed state:** a user invokes `ipython` (which is a new alias in the *python plugin*) and is provided with the proper IPython instance regarding the currently activated virtualenv.
**Example:** the user's default Python is 2.7 with installed IPython 2.7. User activates Python 3.5 virtualenv where he installs IPython 3.5. After activating the environment, one expects `ipython` to run the version of 3.5, which does not happen by default. Instead, IPython 2.7 is used, which in counter-intuitive and often causes problem.
This PR solves this problem.
