New boolean `epy-load-yasnippet-p'

Default is nil

If yasnippet stuff delivered here should be loaded.

Allow not to load in case of error, versions conflicts etc.
keeps alive the remaining.
Merge pull request #131 from pdee/master

In case yasnippet is broken --#119--, setting boolean `epy-load-yasnippet-p' to nil should avoid further disturb