[GH-251] Escape identifier names in MySQL ALTER queries

This issue is created automatically through a Github pull request on behalf of Xowap:

Url: https://github.com/doctrine/dbal/pull/251

Message:

The generated MySQL queries did not properly escape the identifier, thus
making it impossible to use SQL keywords as field/index/table name.

Although this is indeed a quite unwise design choice, this is required
for many legacy applications.

The submitted modifications do cover MySQL only, because it is the one that I need to use. However, I think that all the platforms are flawed with this issue and that a general re-work should be done here.