Map support (kelseyhightower#73)

* support maps with "key1:val1,key2:val2" syntax

* quote the invalid string in error msgs

* document map support in the README

closes kelseyhightower#70

Adding build tags to use `os.LookupEnv` in >=go1.5 (kelseyhightower#65)

* using build tags to allow for os.LookupEnv in >=go1.5

* swapping build tags

* changing to only use os.LookupEnv for appengine

add slice example

add slice example

Merge pull request kelseyhightower#1 from aybabtme/clearer_error

Contextual info to error messages