client, client/test, default: rename page param for LoadPlaylist* to offset

cmd/example, readme, client, client/test: fix readme and example typos, add some benchmarks to client.go, and add some additional comments for LoadPlayer()

formats: fix possible nil pointer dereference when sorting formats over audio quality, and have AudioQuality and VideoQuality map to integers instead of unsigned integers

sig: fix TestCipher relative path

readme: fix url typo

client: add WrapClient helper method to wrap over any arbitrary http client that implements youtube.Transport

all: initial commit