DATAMONGO-804 - Fix default annotation attribute value for repositoryImplementationPostfix().

Changed repositoryImplementationPostfix() in EnableMongoRepositories to "Impl" to be consistent with other EnableXXXRepositories annotations. Note that this change is of rather documenting nature, as the defaulting of the configuration is applied in DefaultRepositoryConfiguration.getImplementationPostfix() in Spring Data Commons.

Original pull request: #99.
DATAMONGO-799 - Fix failing test in MongoTemplateTests on MongoDB 2.5.x.

Generalized exception message matching to reflect the changed exception message in MongoDB 2.5.x that also works with previous versions of MongoDB.

Original pull request: #97.