Add webapi directory / 增加webapi 文件夹

[mibxue]

增加webapi文件夹
用于未来的Web端遥控、遥测、数传数据的处理

[catbraincell]

some issues about webapi implementation:

• Does nodejs backend stable enough for critical applications ?
• Separated frontend / backend implementation ?
• Mysql is appreciated. KS1GCS now have Sqlite integration. ( i think TM and TC data size is small )
• How to store large files ( mostly binary blobs) ? such as image downloaded.
• General authentication framework ? PGP-Key based like ssh ?