fix(security): Ensure the entire secret is random (pquerna#100)

The generated OTP secret can loose entropy due to incomplete reads from
the underlying random generator.
Use `io.ReadFull` to ensure the entire secret buffer is replaced.

Fix URL query encoding (pquerna#78)

* Fix typos

* Fix encoding spaces in issuer name

Update travis for Go 1.15 (pquerna#58)

Merge pull request pquerna#42 from pquerna/pq/expose_setting_secret

Allow calls to {htop,totp}.Generate() to specify the secret

Merge pull request pquerna#31 from champkeh/hotp-digits-check

check hotp key generation opts.Digits

Merge pull request pquerna#19 from pquerna/pq/update_golang_versions

[CI] Update Golang versions on Travis