0.5.0 Prerelease

This first pre-release of BioSequences contains the same functionality as the Seq module in Bio.jl v0.4-0.5, with some minor differences in code, test, and docs organisation, and the addition of a few new doc-strings.

We start at version 0.5 as that was the version at which this package was broken out of Bio.jl. I plan more revisions before a v1 release.