Put apps in a separate directory

[nkryuchkov]

Did you run make format && make check? Yes

Fixes #227.

Changes:

• Put apps in a separate directory
• Fix updater for v version prefix in releases

How to test this PR:

1. In skywire-mainnet's Makefile replace VERSION := $(shell git describe) with VERSION := 0.1.0.
2. Run the generic integration environment.
3. In the shell pane, run curl -k -X POST https://localhost:8080/api/visors/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7/update and wait until it finishes.
4. Check if output equals {"updated":true} and if an update in the VisorA pane finished successfully with no error.
5. After updating, run the previous command, the output should be {"updated":false}.
6. Run make integration-startup-dmsg to add transports and then curl --data '{"recipient":"024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7", "message":"PING!"}' -X POST http://localhost:8001/message to send a message via skychat. The message should appear in VisorAs logs.
7. Check if apps in archives in https://github.com/SkycoinProject/skywire-mainnet/releases/tag/v0.1.1 are located in apps subfolder.

NOTE: The https://github.com/SkycoinProject/skywire-mainnet/releases/tag/v0.1.1 release was created for testing this PR and should be deleted after merging it!

[jdknives]

Works smoothly. Well done.

[Darkren]

Looks great!