Remove standard installation method

Keep only the new package installation method, and discard the old one.
Update installation instructions.

Change internal version to 2.0.0+, and discard the date.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Check Emacs version >= 24 in install-package

Check Emacs version at installation time. Emacs 24 is required.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Move eide-pkg.el into src directory

Since the old installation method has been removed, it is not necessary
to keep eide-pkg.el in a separated directory.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Add a long description (Commentary)

This description is used by Melpa to describe the package.

Update the description in README.rst.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>