Default values, fix smallint & bigint, support mediumint

[nw-b]

• Default values are now supported. (part of Possible to include default values and foreign keys? #5) Non-integer column default values are run through JSON.stringify to ensure they're properly escaped.
• Support mediumint/int3.
• Instead of checking for unsigned in each integer case, just check once after the switch and use the ->unsigned() modifier. (fixes Exporting to Laravel Migration turns unsigned integers into "bigIncrements" #3 due to removal of unsigned int -> bigIncrements typo)
• Add missing alternate type names smallint (in addition to int2) and int1 (in addition to tinyint). (fixes Exporting to Laravel Migration turns unsigned integers into "bigIncrements" #3)

[huyphams]

look good to me, thanks!

[huyphams]

It has some syntax error but I fixed it: 94b2385