fix warning: ensure to check unused variable
fix warning: ISO C90 forbids mixed declarations and code
fix deprecation warning: migrate rb_thread_blocking_region to rb_thread_call_without_gvl
fix some warnings with Ruby 1.8
Merge pull request #415 from tadd/fix-warning

Fix warning
Allow read connection defaults from custom default file and group. #262

For example:
@client = Mysql2::Client.new(:default_file => "/user/.my.cnf", :default_group => "client")
Merge pull request #410 from simi/mysql_read_default_file

Allow read connection defaults from custom default file and group.
Add docs for default_file and default_group