added check to ensure at least one exported field

If no valid env fields are included (i.e. unexported, empty env tags
etc), an error will be returned as the code did no work and this is NOT
what the user expected.
Merge pull request #5 from stretchr/master

Added check to ensure at least one valid field was decoded
add a test for decoding a struct with no tagged fields
Add support to decode time.Duration values from the environment.
Merge pull request #6 from jkakar/duration-values

duration-values
documentation improvements