Make less calls to file-truename.

Makes things very slow when using tramp otherwise, this is a partial
fix. It might be a good idea to not call file-truename here at all on
remote file systems or implement a file-truename cache.
Merge pull request #383 from thomasf/master

Make less calls to file-truename.
Demote errors to messages when trying to load a TAGS file.
Merge pull request #381 from jasonm23/ignore-ctags-load-error

Ignore errors when loading a tags file.
Call error if the tags command failed.

Fixes https://github.com/bbatsov/projectile/issues/377
Merge pull request #379 from Wilfred/master

Call user-error if the tags command failed.
Use `defadvice' rather than the new `nadvice` interface which is only
available in trunk. This fixes f71b5f7.

* projectile.el (projectile-compilation-find-file): Convert to `defavice'.
                (projectile-mode): Use ad-(de)activate.
Merge pull request #386 from daimrod/fix-compilation-find-file

Use `defadvice' rather than the new `nadvice` interface
Mention how to add an alternative key prefix for all Projectile commands