refactor(source/meta): namespace files under browser folders

E.g. move meta/maxthon-inject.js to meta/maxthon/inject.js
feat(maxthon meta): generate def.json using gulp

- add source/meta/maxthon/def.json which is a Lo-Dash template
- in gulp "meta" task compile it to builds/maxthon/def.json
- ignore builds/maxthon/def.json

This is related to #43.
feat(opera meta): generate config.xml using gulp

- add source/meta/opera/config.xml which is a Lo-Dash template
- in gulp "meta" task compile it to builds/opera/config.xml
- ignore builds/opera/config.xml

This finally closes #43.
fix(maxthon): take .gitignore out of extension folder
style(builds/.gitignore): rearrange lines
style(build): rename happy.js to dist.js