Fix type-stability of base64dec
Add base64 test
Speed up base64 encoding/decoding by avoiding use of Iterators
Cache the results of base64dec

This does not help as much as the previous commits, but it shaves another
40% off execution time.
Merge remote-tracking branch 'origin/master' into HEAD

Conflicts:
	src/Codecs.jl
Merge pull request #7 from timholy/pull-request/874247f0

Pull request/874247f0