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

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

[coveralls]

Coverage remained the same at 92.675% when pulling a6b9a4f on galaunay:Add-variable-explorer into c8a7e09 on jorgenschaefer:master.