Added mysql support for

Folks: my goal is to use phinx to create a set of migration scripts for a database of 164 tables to be able to then migrate all the tables to a new DB, do a diff and not see any discrepancies between the old and the new.  
double. These changes have allowed that to happen.

Datatypes added:
longblob
mediumtext
tinyint
bit
char

Corrected/added support for default column value for the mysql "empty string" value
Corrected Unique index handling...was always true no matter the value.
Corrected foreign key constraint name handling...did not provide a way to name a foreign key contraint.

My client would appreciate being able to update with composer.

I noticed that github shows spacing issues. I cannot see these descrepancies in my dev environment, so if you have any comments, please direct me.  (4 spaces per indent). 

Signed-off-by: Jonathan Galpin <jonathan@iqzero.net>