Add a variable explorer to Elpy

[galaunay]

PR Summary

Add a variable explorer to Elpy.

This allows to have a side-buffer displaying the variables currently defined in the associated python shell.

Todo list

• [x] Add the possibility to see a variable value in detail in another buffer
• [ ] Automatically refresh the variable explorer when executing code ?
• [x] Add keybinding in the variable explorer buffer
• [x] Add a keybinding for the variable explorer
• [x] Commits respect our guidelines
• [x] Tests are passing properly (see here on how to run Elpy's tests)
• [x] Tests has been added to cover the change
• [x] The documentation has been updated

[coveralls]

Coverage remained the same at 92.675% when pulling a6b9a4fc5b929017110f634df779085b64f5f67f on galaunay:Add-variable-explorer into c8a7e093c7c2191d4d417448340372279b120673 on jorgenschaefer:master.