This hasn't been enabled in a while due to gRPC limitations

A config file is not required - don't error if one doesn't exist.

Don't overwrite default options with an empty string.

- Add `fast_finish` to travis config
- Specify that Go 1.9 is supported
- Additional error checking which was previously missed
- Make the migrations test better
-

In retrospect a primary key that has no relation to the certificate is preferred to using the certificate KeyID. The KeyID is also very large for a primary index.
This is a moderately tricky migration, especially for SQLite which has no means of altering the table in this fashion - it involves creating the new table and copying the data.
Order of commands also matters - index names are global in SQLite, so the `idx_expires_at` index needs to be created at the correct stages.

For MySQL migration the necessary steps are run as a single alter statement to minimise the risk of leaving the migration in an incomplete state if anything aborts.

When tested on a table with 250,000 rows (MySQL 5.7) the migration took 3 seconds to complete. As certificates will be requested infrequently the risk of prolonged locking is minimal.

Static content generator finds the root of the git repo and executes from there. Packr will scan subdirs for boxes to be packed.

Return an error if the store isn't known, instead of defaulting to a mem store

It's currently hard to make changes to the database schema. Use sql-migrate to make incremental changes.
Stop hard-coding the database name (the default is still "certs" for backward-compatibility)
The `automigrate()` function will automatically run pending migrations.
Use a different migration directory per database driver. This carries a cost of duplication, but is easier than creating migrations which will cleanly execute in both SQLite and MySQL.
Migrations are shipped using the packr utility.

Makefile:
- Use goimports to format
- Regenerate static files
- Check that the git branch is clean

README:
- Remove section on Docker. To be re-added.
- Link to releases

* Support a message to be passed and logged from client to server

* enables saving private keys

* renames public_file_prefix to key_file_prefix and updates its docs to better reflect the changes