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
fix warning: int is not a pointer.
fix warning: object that pointed is const.
Merge pull request #426 from tadd/fix-warning-2

Fix warning again
Test connecting to DB with numeric-only name
Per tests from BanzaiMan, convert non-nil values before passing them to the C connect function.
Merge pull request #427 from sodabrew/banzaiman_stringify

Connect to DB whose name consists entirely of numerics by coercing connection data types on the Ruby side.
Eschew use of `sed`

At least on the Mac, the `sed` command silently fails, not replacing
`LOCALUSERNAME`.
There, a correct invocation is something like:
```
sh "sed -i '' -e 's/LOCALUSERNAME/#{ENV['USER']}/' #{dst_path}"
```

Rather than comprehensively account for differences in `sed` on various
platforms, use Ruby's File class to manipulate the files.
Merge pull request #429 from BanzaiMan/no_sed

Eschew use of `sed`