[WIP] Add BIP 151 support

[droark]

Initial step: In Git, submodules and subtrees seem to have fundamental issues with each other that prevent subtrees from being used alongside submodules. Delete the FCGI submodule, re-add it as a subtree, and add Jonas Schnelli's ChaCha20Poly1305 library.

Much work remains to be done. The list includes but is not limited to:

• Integrate Jona's library into Armory's make system and confirm that the library is built.
• Add a library that can handle ECDH (libbtc, which will be used by Armory eventually and contains libsecp256k1, where the "experimental" ECDH code looks fine to me and is used in production environments already).
• Confirm that Armory code can call libchacha20poly1305 and libsecp256k1.
• Implement BIP 151 in Armory.
• Confirm that the Armory build system properly builds the test and benchmark binaries if requested.
• Because BIP 151 support in Core may not come for awhile (0.18 is my guess), look into ways to test Armory's BIP 151 capabilities. This may include writing or modifying a Wireshark plugin, or using some other tool to inject raw packets into Armory.
• Any miscellaneous cleanup.
• (TASK FOR GOATPIG?) Integrate the library into the Windows build system.
• (RECOMMENDED) Further cleanup of Armory's build system as needed.

[droark]

Confirmed that the library compiles on Ubuntu 17.10. Will try macOS later. Windows will have to wait.

I also want to point out one specific change I made in Makefile.am. ArmoryDB was listed under noinst_PROGRAMS. This doesn't make sense to me. If make install is being invoked, you want ArmoryDB to be part of the install. So, I fixed this. While I included it as a possible task, cleaning up the Make system is probably best done as a separate PR. Not that the system is terrible now but it could use some cleanup, not to mention I've heard people complain that proper Armory packages can't be built.

[droark]

Per our discussion, I'll be backing out the libsecp256k1 commit and adding libbtc, which includes libsecp256k1. libbtc technically isn't part of this commit but we're planning to use it as a drop-in replacement for Crypto++ eventually (0.98???). We might as well get it in the build system now.

[droark]

libbtc has been added. However, for now it may not be possible to build libbtc on Windows. This PR is designed to allow the code to compile using Visual Studio 2015. It'll need a push (and a rebase) if it's going to be committed, otherwise somebody will have to fix everything up down the road.

For now, I'm just going to build libbtc on Linux and macOS. If it becomes an issue, I can just compile libsecp256k1 until we're ready for Armory to use libbtc and replace Crypto++.

[droark]

Incoming turbulence. Rebase is doing some weird things. Am thinking it's best to just delete this branch and start fresh. I'll try to avoid having to close this PR.