add support to create foreign pointcloud objects

[ldgeo]

This PR adds new endpoints to create foreign server, tables and schema:

• GET /foreignpc/drivers -> list of available multicorn wrappers
• GET /foreignpc/servers -> list of foreign servers with their options
• POST /foreignpc/servers -> add a foreign server
• POST /foreignpc/table -> add a foreign table
• POST /foreignpc/schema -> add a rosbag schema

Fixes #30

[elemoine]

For reference here's how to use httpie to create a foreign table:

http POST localhost:5000/foreignpc/table X-API-KEY:aaaabbbbcccc server=sbetserver srid:=0 table=public.mysbet3 options:='{"sources": "/home/elemoine/src/fdw-li3ds/test/data/sbet/sbet.bin", "patch_size": 100}'

[elemoine]

I just tested the /schema endpoint to create foreign tables from a rosbag file. It works as expected.

Create the rosbag server:

$ http POST localhost:5000/foreignpc/servers X-API-KEY:aaaabbbbcccc name=rosbagserver driver=fdwli3ds.Rosbag options:='{"rosbag_path": "/home/elemoine/src/fdw-li3ds/test/data/rosbag/"}'

Import the foreign schema:

$ http POST localhost:5000/foreignpc/schema X-API-KEY:aaaabbbbcccc server=rosbagserver schema=session8 rosbag=session8_section0_1492648601948956966_0.bag

[mbredif]

Great!