Use VARCHAR(255) for Firebird string default

The Firebird generator was using BLOB SUB_TYPE TEXT as the default type
for AsString(). I've changed it to use VARCHAR(255) instead since
this is more likely what is actually desired.

column.AsString() => VARCHAR(255)
column.AsString(100) => VARCHAR(100)
column.AsString(10000000) => BLOB SUB_TYPE TEXT
Use more accurate max text size

Firebird can handle up to 32GB but we're limited to int.MaxValue.
Improve text size comment
Allow data to be inserted using a dictionary and refactor existing code closes #458
Update MySqlTypeMap.cs

version 1.1.2 causes error when migrating 'versioninfo' table by adding a 'description' column on MySQL database.
This error caused by method 'Up' in 'VersionDescriptionMigration' class. The method calls .AsString(int.MaxValue) which is beyond 
definition in MySqlTypeMap class
Merge pull request #484 of kingpong/firebird-default-string

* kingpong-firebird-default-string:
  Update MySqlTypeMap.cs
  Allow data to be inserted using a dictionary and refactor existing code closes #458
  Improve text size comment
  Use more accurate max text size
  Use VARCHAR(255) for Firebird string default