Fix file reading and seeking.
Merge branch 'master' of github.com:kvesteri/flask-storage
NotImplemented -> NotImplementedError.
Open S3 files before reading them.
Write is not implemented for S3 files.
Make S3 file reading work and declare seeking as not implemented.

The keys' read method starts from where it left, but has no way of seeking. The
only way to seek would be to store the entire file contents in locally or in
memory and read from there instead.
Bump version.
Fix Cloud file reading.
Fix Amazon tests.
Add tests for reading and opening Amazon files.