Switch to rolling release

The `go1` tag caused a lot of confusion and did more harm than good.
In order to make `go get` work as expected and reduce the amount of duplicate bug reports, we switch back to a "rolling release". `go get` will install the current master branch again.
But we will continue to create branches for stable versions.
CHANGELOG: rolling release
Merge pull request #243 from go-sql-driver/rolling

Switch to rolling release
Registration of custom dial functions
remove buffer indirection
Also remove MySQLDriver indirection
Merge pull request #245 from go-sql-driver/dial

Registration of custom dial functions
Merge pull request #246 from go-sql-driver/buffer

remove buffer and driver indirections
Fix import not used.
Merge pull request #247 from filipenos/master

appengine: fix import not used.
Release v1.2
Merge pull request #248 from go-sql-driver/v1.2

Release v1.2
support microseconds with MySQL 5.7+

passes all tests except TIME(1) -> string in binary protocol.
TIMESTAMP support with microsecond resolution is still incomplete.
all tests passing