Rescue encryptor errors

This change set handles the new exception classes introduced in https://github.com/attr-encrypted/encryptor/pull/13 . This should allow clients of AttrEncrypted to rescue any currently known exception coming out of Encryptor (and by extension OpenSSL) in a sane manner. The tests for this won't pass on Jenkins until a new version of `Encryptor` is released with these changes but it is easy to test locally  if both gems are checked out.

Gemfile diff to test locally:

```diff
diff --git a/Gemfile b/Gemfile
index 6f1fe01..896b2ca 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,10 +1,10 @@
 source "http://rubygems.org"

 # if you're also working on encryptor use the local copy
-#gem 'encryptor', :path => '../encryptor'
+gem 'encryptor', :path => '../encryptor'

 # if you are just working on attr_encrypted use this version
-gem 'encryptor', :git => 'https://github.com/attr-encrypted/encryptor'
+#gem 'encryptor', :git => 'https://github.com/attr-encrypted/encryptor'

 gemspec
```